Powertrain Control Module (PCM) General Information

Powertrain

The powertrain used in this vehicle consists of a dual overhead cam engine mated to a 4T80-E electronically controlled transaxle. It has controls to reduce exhaust emissions while maintaining excellent driveability and fuel economy. The Powertrain Control Module (PCM) is the heart of this control system.

The control module refers to the powertrain control module (PCM). The control module is designed to maintain exhaust emission levels to Federal or California standards while providing excellent driveability and fuel efficiency. Review the components and wiring diagrams in order to determine which systems are controlled by each specific control module. The control module monitors numerous engine and vehicle functions. The control module controls the following operations:

Control Module Function

The control module supplies a buffered voltage to various sensors and switches. The input and output devices in the control module include an analog to digital converters, signal buffers, counters, and special drivers. The control module controls most components with electronic switches which complete a ground circuit when turned ON. These switches are arranged in groups of 4 and 7 called one of the following:

The surface mounted quad driver module can independently control up to 4 outputs (control module) terminals. The output driver modules can independently control up to 7 outputs. Not all outputs are always used.

Torque Management

Torque Management is a function of the PCM that reduces engine power under certain conditions. Torque management is performed for three reasons:

1. To prevent overstress of powertrain components.
2. To limit engine power when brakes are applied.
3. To prevent damage to the vehicle during certain abusive maneuvers.

The PCM uses manifold vacuum, intake air temperature, spark retard, engine speed, engine coolant temperature, A/C clutch status, and EGR valve position to calculate engine output torque. It then looks at torque converter status, transaxle gear ratio, and brake switch inputs and determines if any torque reduction is required. If torque reduction is required, the PCM retards spark as appropriate to reduce engine torque output. In the case of abusive maneuvers, the PCM may also shut off fuel to certain cylinders to reduce engine power.

There are five instances when engine power reduction is likely to be experienced

In the first two instances, the driver is unlikely to even notice the torque management actions. In the other cases, engine power output will be moderate at full throttle.

When the PCM determines that engine power reduction is required, it calculates the amount of spark retard necessary to reduce power by the desired amount. This spark retard is then subtracted from the current spark advance. In the case of abusive maneuvers, the fuel injectors for cylinders 1, 4, 6, and 7 will also be disabled for a period of time.

Traction Control

Traction Control is a function of the PCM and the EBTCM that reduces front wheel slip during acceleration by applying the front brakes and reducing engine power. Refer to Section 5 for an explanation of the EBTCMs role in traction control. The PCM continuously sends out a PWM signal that indicates the torque output of the powertrain. This signal, referred to as the Delivered Torque signal, is used by the EBTCM to determine what action is required when it sees the front wheels slipping. The EBTCM may decide to apply the front brakes only or apply the front brakes and request reduced torque output from the powertrain. The EBTCM requests reduced torque using another PWM signal. This signal, referred to as the Desired Torque signal, is used by the PCM to determine if the EBTCM is requesting reduced torque output from the powertrain. If the EBTCM requests reduced torque, the PCM will disable between one and seven fuel injectors to achieve this.

Desired Torque will normally be a 90 percent duty cycle signal to the PCM. When the EBTCM decides to request reduced engine power, it decreases the duty cycle of the Desired Torque signal by the amount of torque reduction required (90 percent duty cycle means no torque reduction, 10 percent duty cycle means 100% torque reduction). The PCM responds by shutting off fuel to one or more cylinders depending on the percent torque reduction requested. The PCM will not shut off any fuel injectors if any of the following conditions are present

The disabled fuel injectors will be re-enabled one by one as the need for traction control ends.

Several DTC(s) disable traction control when set. They will also trigger a TRACTION OFF light or message. The PCM traction control override also disables traction control and triggers the message. To diagnose a Traction Off light/message, diagnose any DTC(s) set first. Then check the traction control override and, if active, deactivate the override. If the TRACTION OFF light/message is still present, refer to Section 5 for further diagnosis.

Service Engine Soon Malfunction Indicator Lamp (MIL)

The SERVICE ENGINE SOON Malfunction Indicator Lamp (MIL) is located in the Instrument Panel Cluster (IPC). The SERVICE ENGINE SOON MIL is controlled by the PCM through CKT 419 and is used to indicate that the PCM has detected a problem that affects vehicle emissions, may cause powertrain damage, or severely impacts driveability. Refer to DTC P1641 for diagnosis of Service Engine Soon MIL.

How To Use This Section

This section is designed as the starting point for all powertrain diagnosis. It contains a basic description of this powertrain, detailed information on the PCM, on-board diagnostics, and selected other components as well as references to other sections for detailed information on powertrain subsystems not covered here. Before diagnosing this vehicle, you should have read and be familiar with

Once familiar with this information, you may proceed with diagnosis.

Where To Start Diagnosis

If you are reasonably certain that the problem is powertrain related, the first place to start is with the POWERTRAIN OBD SYSTEM CHECK in this section.

While performing diagnosis, you may need to access certain components. Be sure to read the On-Vehicle Service procedures for those components before accessing them.

During diagnosis or if a wiring problem is found, you may need to refer to Section 8. Section 8 contains complete information on this vehicles wiring including:

After Diagnosis Is Complete

After diagnosing a problem, in addition to repairing the discovered fault, it is a good idea to clear any DTC(s) set and verify proper operation. To clear DTC(s), refer to Clearing Diagnostic Trouble Codes later in this section. To verify a repair, you must duplicate the conditions when the customers complaint occurred or if a DTC was set, you must duplicate the Test Conditions for that DTC and see if the DTC sets. If the DTC runs and passes, chances are good that the problem is repaired.

Diagnostic Procedure Use

The diagnostic procedures used in this section are designed to find and repair powertrain related problems. The general approach is to find the appropriate diagnosis for a problem with five basic steps described below.

1. Understand the customers complaint. It is critical that the technician understand what the customers complaint is. Failure to understand this may lead to misdiagnosis or unnecessary diagnosis. Among other things, the technician must know whether the condition is present at all times, only under certain circumstances, or truly intermittent (random). This will assist the technician in duplicating and diagnosing the problem. Another reason the technician must understand the customers complaint is so the technician may determine whether the complaint requires service or is normal vehicle operation. Trying to diagnose a complaint that is normal will waste time and may result in unnecessary service.
2. Are Diagnostics working properly? Use the Powertrain OBD System Check. This is the starting point for the diagnostic procedure, always begin here.
3. Are DTC(s) displayed? If a DTC is identified by diagnostics, the Powertrain OBD System Check will direct you to the appropriate table.
4. Is the customers complaint related to a specific powertrain subsystem? If no related DTC(s) are set, the next quickest way to locate the problem is to narrow it down to a specific powertrain subsystem. If a specific subsystem can be pinpointed as the cause, it is easier to diagnose.
5. Is the problem powertrain related? Some customer complaints may appear to be powertrain related but are actually caused by other vehicle systems.

Basic Knowledge Required

You must be familiar with some of the basics to use this section of the Service Manual. They will help you to follow diagnostic procedures in this section.

Notice: Lack of basic knowledge of this powertrain when performing diagnostic procedures could result in incorrect diagnostic performance or damage to powertrain components. Do not attempt to diagnose a powertrain problem without this basic knowledge.

Basic Electrical Circuits

You should understand basic electricity and know the meaning of voltage (volts), current (amps), and resistance (ohms). You should understand what happens in a circuit with an open or a shorted wire and you should be able to identify a shorted or open circuit using a DMM. You should be able to read and understand a wiring diagram.

Use Of Digital Multimeter (DMM)

You should be familiar with the Digital Multimeter (DMM), particularly essential tool J 39200. You should be able to use the meter to measure voltage (volts), resistance (ohms), current (amps), capacitance (farads), intermittents (min/max) and frequency (Hertz).

Use Of Circuit Testing Tools

You should only use a test light when a diagnostic procedure refers to it's use. You should know how to use jumper wires to test components and allow DMM readings without damaging terminals. You should know how to use Connector Test Adapter Kit, J 35616 and use it whenever diagnostic procedures call for front probing any connector.

PCM Service Precautions

The PCM is designed to withstand normal current draws associated with vehicle operations, however, care must be used to avoid overloading any of these circuits. In testing for opens or shorts do not ground or apply voltage to any of the PCM circuits unless instructed to do so by the diagnostic procedures. These circuits should only be tested using the Digital Multimeter (J 39200).

Whenever a PCM removal and replacement is performed, follow the procedures in this section.

Electrostatic Discharge Damage

Notice: In order to prevent possible Electrostatic Discharge damage to the PCM, Do Not touch the connector pins or the soldered components on the circuit board.

The electronic components used in the control systems are often designed in order to carry very low voltage. The electronic components are susceptible to damage caused by electrostatic discharge. Less than 100 volts of static electricity can cause damage to some electronic components. There are several ways for a person to become statically charged. The most common methods of charging are by friction and by induction. An example of charging by friction is a person sliding across a car seat. Charging by induction occurs when a person with well insulated shoes stands near a highly charged object and momentarily touches ground. Charges of the same polarity are drained off, leaving the person highly charged with the opposite polarity. Static charges can cause damage. Use care when handling and testing the electronic components.

On-Board Diagnostic (OBD) System

This vehicle is equipped with an extensive on-board diagnostic system (PCM) which is capable of detecting and compensating for faults and alerting the operator.

Emissions Diagnosis For State I/M Programs

This OBD II equipped vehicle is designed to self-diagnose conditions that could lead to excessive levels of the following emissions:

Should this vehicles onboard diagnostic system (PCM) detect a condition that could result in excessive emissions, the PCM turns ON the MIL and stores a DTC associated with the condition. Follow the diagnostic procedure used for any driveability condition when diagnosing a condition reported from a state I/M program.

Aftermarket (ADD-ON) Electrical And Vacuum Equipment

Aftermarket (Add-on) Electrical and Vacuum Equipment is defined as any equipment installed on a vehicle after leaving the factory where the vehicle was originally assembled that connects, in any way, to the vehicles electrical or vacuum systems. No allowances have been made in the design of this vehicle for this type of equipment. Therefore, addition of aftermarket equipment must be done with the utmost care for the vehicle.

Notice: In order to prevent damage, do not add on any Add-On vacuum operated equipment to this vehicle.

Notice: In order to prevent damage to the vehicle, only connect the Add-On electrical operated equipment to the electrical system of the vehicle at the battery (power and ground).

Add-on electrical equipment, even when installed to these strict guidelines, may still cause the powertrain system to malfunction. Therefore, the first step in diagnosing any powertrain problem is to eliminate any and all aftermarket electrical equipment from the vehicle. Once this is done, if the problem still exists, it may be diagnosed in the normal manner.

DTC(s)

DTC(s), when set, indicate that the PCM has detected a malfunction in a particular circuit or system. The PCM is programmed with routines or checks that it follows only under prescribed conditions (called Test Conditions). When these conditions exist, the PCM checks certain circuits or systems for a malfunction. These checks are called failure conditions. When the failure conditions are true, a DTC is set. Some DTC(s) alert the driver through the SERVICE ENGINE SOON MIL or a message. Other DTC(s) dont trigger a driver warning. Refer to the DTC TYPE TABLE for a complete list of PCM DTC(s) and what driver alerts they trigger. The PCM also saves data and input parameters when most DTC(s) are set.

Unleaded Export Vehicle DTC Type Information

The service information contained in this manual refers to the domestic calibration package - United States, Puerto Rico, U.S. Virgin Islands, Caribbean and Guam. The Canadian and Unleaded Export calibration package (K 29) is referenced in this service manual in the DTC Type Table only. For the Unleaded Export calibration package some DTCs have been disabled and others have changed types (Type A to Type D, etc.). If a DTC has changed type or is disabled for the Unleaded Export calibration package, this change is NOT reflected in the service information on the DTCs diagnostic support information page. The Unleaded Export calibration package consists of vehicles sold in: Canada, Asia, Korea, Taiwan, Japan, Europe, Israel, Mexico, Iceland, South and Central America and Fleet/Govt./Military.

DTC Status

This selection will display any DTC(s) that have not run during the current ignition cycle or have reported a test failure during this ignition cycle up to a maximum of 40 DTC(s). DTC tests which run and pass will cause that DTC number to be removed from this scan tool display.

Fail This Ign. (Fail This Ignition)

This selection will display all DTC(s) that have failed during the present ignition cycle.

Last Test Fail

This selection will display only DTC(s) that have failed the last time the test ran. The last test may have ran during a previous ignition cycle if an A or B type DTC is displayed. For type C DTC(s), the last failure must have occurred during the current ignition cycle to appear as Last Test Fail.

MIL Request

This selection will display only DTC(s) that are requesting the MIL. Type C DTC(s) cannot be displayed using this option. This selection will report type B DTC(s) only after they have requested the MIL. (failed twice).

Not Run SCC (Not Run Since Code Clear)

This selection will display up to 40 DTC(s) that have not run since DTC(s) were last cleared. Since any displayed DTC(s) have not run, their condition (passing or failing) is unknown.

Test Fail SCC (Test Failed Since Code Clear)

This selection will display all active and history DTC(s) that have reported a failure since the last time DTC(s) were cleared. DTC(s) that last failed over 40 warm-up cycles will not be displayed.

History Code

This selection will display only DTC(s) that are stored to the PCMs history memory. It will not display type B DTC(s) that have not requested the MIL. It will display all type A DTC(s) and type B DTC(s) (which have requested the MIL) that have failed within the last 40 warm-up cycles. In addition, it will display all type C DTC(s) that have failed within the last 40 warm-up cycles.

Diagnostic Trouble Code Diagnosis

This section uses diagnostic charts, wiring diagrams, and descriptive text to direct you in performing specific tests to locate and repair the problem. The diagnostic chart is a step by step procedure to determine the circuit or component that is the source of the problem. The wiring diagrams and text describing the system, test conditions, failure conditions, and actions taken when a DTC is set are located on the diagnostic information support page before the table.

Diagnostic Code Display

DTC(s) can only be displayed with the use of a scan tool.

Clearing Diagnostic Trouble Codes

When using the scan tool to clear DTC(s) select Clear Info. Before clearing DTC(s) the scan tool has the capability to save any data stored with the DTC(s) and then display that data at a later time, (refer to Capture Info.). Once a problem has been corrected and verified it is a good idea to clear DTC(s) so that any future service work is not needlessly confused.

Many PCM DTC(s) have complex test and failure conditions. Therefore, simply clearing DTC(s) and watching to see if the DTC sets again may not indicate whether a problem has been corrected. To verify a repair after it is complete, you must look up the test conditions and duplicate those conditions. If the DTC runs and passes, chances are good that the problem is fixed.

Capturing DTC Info (Capture Info)

Selecting this option on the scan tool allows the technician to record the Freeze Frame and Failure Records that may be stored in the PCMs memory. This can be useful if the PCM or battery must be disconnected and later review of the stored information may be desired.

PCM Snapshot

The PCM snapshot feature is designed to assist in diagnosis of intermittent or unrepeatable problems. When a PCM snapshot is taken, all or some of the PCMs data and input parameters from that moment in time are stored for retrieval and for use in diagnosing a problem. PCM snapshots are taken in three instances; when most DTCs are set, when Customer Snapshot is initiated, or when a Snapshot is commanded with the scan tool.

All PCM snapshot data may be accessed through the Freeze Frame / Failure Records option on the diagnostic tool.

Customer Snapshot

The Northstar powertrain has an unique feature that allows a PCM snapshot to be recorded without using a scan tool by simply pressing two buttons on the Climate Control Center (CCC). This feature, called Customer Snapshot, records a complete set of PCM data parameters and inputs when the OFF and FRONT DEFROST (Analog dash) or OFF and WARMER (Digital dash) buttons on the CCC are pressed simultaneously. Customer Snapshot only works when the vehicle is NOT in the diagnostics mode. This feature can be very useful when it is difficult to reproduce a condition. When this occurs, the owner or driver may be instructed to take a Customer Snapshot while the condition is present and return to the dealer for diagnosis when convenient. A Customer Snapshot may be taken anytime the ignition is on and the vehicle is not in on-board diagnostics. While this is true, pressing the correct buttons simultaneously for one second will cause a snapshot to be taken and the SERVICE ENGINE SOON MIL to be turned ON for 2 seconds as confirmation that a snapshot was taken.

This data must be retrieved by using a diagnostic tool. The Customer Snapshot data will be overwritten if another Customer Snapshot is taken or if a scan tool snapshot is taken.

PCM Snapshot On Code Set (Freeze Frame, Failure Records)

When a PCM DTC sets, the PCM does several things. Among them is to save useful data and input parameter information for service diagnosis. This information is referred to as Freeze Frame, Failure Records. You will see references to these in many PCM DTC trouble charts because this information can be useful in pinpointing a problem even if the problem is not present when the vehicle is in the service bay.

Freeze Frame, Failure Records save relevant data parameters with a DTC snapshot. Which data parameters are saved varies based on the DTC set. Transaxle DTC snapshots generally contain transaxle related data parameters while fuel DTC snapshots generally contain fuel related data parameters. If several DTC(s) are set, snapshots for the three most recently set DTC(s) will be saved.

Freeze Frame, Failure Records data may be retrieved through the DTC menu on scan tool. If more than one DTC is set review the odometer or engine run time data located in the Freeze Frame, Failure Records info to determine the most current failure.

Keep in mind that once Freeze Frame or Failure Record is selected, the parameter and input data displayed will look just like the normal PCM data except the parameters will not vary since it is displaying recorded data.

PCM Snapshot Using A Scan Tool

PCM snapshot data may also be taken and retrieved using a scan tool. These parameters may then be viewed right away.

Important: Taking a PCM snapshot will erase any Customer Snapshot data previously stored.

Keep in mind that once Snapshot is triggered, the parameter and input data displayed will look just like the normal PCM parameter and input data with the same parameter numbers except it will not vary since it is displaying recorded data.

Diagnosing Intermittents Using PCM Snapshots

PCM snapshots may be very useful in diagnosing powertrain problems. Data stored through Customer Snapshot, by the scan tool, or when a PCM DTC was set may be retrieved and used to determine if a particular input was incorrect when the problem occurred. Many of the PCM DTC diagnostic tables will use Freeze Frame, Failure Records information to pinpoint a problem if it is not current. For example, the diagnostic table for DTC P1810 uses the gear ratio snapshot data parameter to indicate what gear the transaxle was in when the DTC set. This information may be used along with the transaxle pressure switch indication to zero in on the circuit(s) that are intermittently shorted or open. In order to use Customer Snapshot data or PCM snapshot data that was recorded by the scan tool, you must first understand what problem the driver was trying to capture in the snapshot. Once this is understood, any appropriate parameters may be retrieved and any abnormal parameter readings used to pinpoint what sensor or system is causing the problem. For example, say a vehicle comes in with an intermittent missing engine condition and a Customer Snapshot was taken while the engine was running poorly. Several data parameters may be retrieved and an abnormal Extended Travel Brake switch parameter noted. The MAP sensor, ECT sensor, IAT sensor, and knock sensor parameters may also be compared to normal values of those parameters and any values that are out of the normal range investigated. Also, fuel injector pulse width or spark advance may be retrieved. This may indicate a fueling or ignition irregularity that should be investigated. With the Extended Travel Brake switch applied all the time, you determine the vehicle is incorrectly running in Torque Management during acceleration. While this method may result in increased diagnosis time, it also results in less actual repair time and fewer service comebacks.

Powertrain Control Module (PCM) Emissions and OBD II

System Status and Drive Cycle For Satisfying the Federal Inspection/Maintenance (I/M 240) Regulations

The system status selection is included in the scan tool System Info menu.

Several states require that the I/M 240 (OBD ll system) pass on-board tests for the major diagnostics prior to having a vehicle emission inspection. This is also a requirement to renew license plates in some areas.

Using a scan tool, the technician can observe the system status, complete or not complete, in order to verify that the vehicle meets the criteria which complies with local area requirements. Using the system status display, any of the following systems or a combination of the systems may be monitored for I/M readiness:

Important: The system status display indicates only whether or not the test has been completed. The system status display does not necessarily mean that the test has passed. If a Failed Last Test indication is present for a DTC associated with one of the above systems, that test is failed. Diagnosis and repair is necessary in order to meet the I/M 240 requirement. Verify that the vehicle passes all of the diagnostic tests associated with the displayed system status prior to returning the vehicle to the customer. Refer to the Typical OBD II Drive Cycle table to use as a guide to complete the I/M 240 system status tests. More than one drive cycle may be needed.

Following a DTC info clear, the system status will clear only for the systems affected by any DTCs stored. Following a battery disconnect or a control module replacement, all of the system status information will clear.

Typical OBD II Drive Cycle

Diagnostic Time Schedule for I/M Readiness
Vehicle Drive Status	What is Monitored?
Cold Start, coolant temperature less than 50°C (122°F)	--
Idle 2.5 minutes in Drive (Auto) Neutral (Man), A/C and rear defogger ON	HO2S Heater, Misfire, Secondary Air, Fuel Trim, EVAP Purge
A/C off, accelerate to 90 km/h (55 mph), 1/2 throttle.	Misfire, Fuel Trim, Purge
3 minutes of Steady State - Cruise at 90 km/h (55 mph)	Misfire, EGR, Secondary Air, Fuel Trim, HO2S, EVAP Purge
Clutch engaged (Man), no braking, decelerate to 32 km/h (20 mph)	EGR, Fuel Trim, EVAP Purge
Accelerate to 90-97 km/h (55-60 mph), 3/4 throttle	Misfire, Fuel Trim, EVAP Purge
5 minutes of Steady State Cruise at 90-97 km/h (55-60 mph)	Catalyst Monitor, Misfire, EGR, Fuel Trim, HO2S, EVAP Purge
Decelerate, no breaking. End of Drive Cycle	EGR, EVAP Purge
Total time of OBD II Drive Cycle 12 minutes	--

Primary System Based Diagnostics

There are primary system-based diagnostics which evaluate the system operation and their effect on vehicle emissions. The primary system-based diagnostics are listed below, with a brief description of the diagnostic functionality.

Oxygen Sensor Diagnosis

Diagnose the Fuel Control Heated Oxygen Sensors (Bank 1 HO2S 1 and Bank 2 HO2S 1) for the following conditions:

Diagnose the Catalyst Monitor Heated Oxygen Sensors (Bank 1 HO2S 2 and Bank 1 HO2S 3) for the following functions:

Fuel Control Heated Oxygen Sensors (Bank 1, HO2S 1) (Bank 2, HO2S 1)

The main function of the fuel control heated oxygen sensor is to provide the control module with exhaust stream information in order to allow proper fueling and maintain emissions within the mandated levels. After the sensor reaches the operating temperature, the sensor generates a voltage inversely proportional to the amount of oxygen present in the exhaust gases.

The control module uses the signal voltage from the fuel control heated oxygen sensors in a closed loop in order to adjust the fuel injector pulse width. While in a closed loop, the Control Module can adjust fuel delivery in order to maintain an air to fuel ratio which allows the best combination of emission control and driveability.

If the oxygen sensor pigtail wiring, connector or terminal are damaged, replace the entire oxygen sensor assembly. Do not attempt to repair the wiring, connector, or terminals. In order for the sensor to function properly, the sensor must have a clean air reference provided to it. This clean air reference is obtained by way of the oxygen sensor wires. Any attempt to repair the wires, connectors or terminals could result in the obstruction of the air reference. Any attempt to repair the wires, connectors or terminals could degrade oxygen sensor performance.

Catalyst Monitor Heated Oxygen Sensors (Bank 1 HO2S 2 and Bank 1 Sensor 3)

In order to control emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NOx), the system uses a three-way catalytic converter. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it to nitrogen.

The Control Module has the ability to monitor this process using the Bank 1 HO2S 2 and the Bank 1 HO2S 3 heated oxygen sensors. The Bank 1 HO2S 2 sensor produces an output signal which indicates the amount of oxygen present in the exhaust gas entering the three-way catalytic converter. The Bank 1 HO2S 3 sensor produces an output signal which indicates the oxygen storage capacity of the catalyst; this in turn indicates the catalyst's ability to convert exhaust gases efficiently. If the catalyst is operating efficiently, the Bank 1 HO2S 2 signal will be far more active than that produced by the Bank 1 HO2S 3 sensor.

In addition to catalyst monitoring, the Bank 1 HO2S 3 heated oxygen sensor has a limited role in controlling fuel delivery. If the Bank 1 HO2S 3 signal indicates a high or low oxygen content for an extended period of time while in a closed loop, the Control Module adjusts the fuel delivery slightly in order to compensate.

Catalyst Monitor Diagnostic Operation

The OBD II catalyst monitor diagnostic measures oxygen storage capacity. In order to do this, the heated sensors are installed before and after the Three-Way Catalyst (TWC). Voltage variations between the sensors allow the control module to determine the catalyst emission performance.

As a catalyst becomes less effective in promoting chemical reactions, the catalyst's capacity to store and release oxygen generally degrades. The OBD II catalyst monitor diagnostic is based on an correlation between conversion efficiency and oxygen storage capacity.

A good catalyst (e.g. 95% hydrocarbon conversion efficiency) shows a relatively flat output voltage on the post-catalyst Heated Oxygen Sensor (HO2S). A degraded catalyst (65% hydrocarbon conversion) shows a greatly increased activity in output voltage from the post catalyst HO2S.

The post-catalyst HO2S 2 is used to measure the oxygen storage and release capacity of the catalyst. A high oxygen storage capacity indicates a good catalyst; low oxygen storage capacity indicates a failing catalyst. The TWC and both the Pre and Post Converter HO2S must be at operating temperature in order to achieve correct oxygen sensor voltages like those shown in the Post-Catalyst HO2S 3 Outputs graphic.

The catalyst monitor diagnostic is sensitive to the following conditions:

Exhaust system leaks may cause the following results:

Some of the contaminants that may be encountered are phosphorus, lead, silica, and sulfur. The presence of these contaminants prevents the TWC diagnostic from functioning properly.

Three-Way Catalyst Oxygen Storage Capacity

The control module must monitor the three-way catalyst (TWC) system for efficiency. In order to accomplish this, the control module monitors the pre-catalyst and post-catalyst oxygen sensors. When the TWC is operating properly, the post-catalyst (2) oxygen sensor will have significantly less activity than the pre-catalyst (1) oxygen sensor. The TWC stores the oxygen as needed during the normal reduction and oxidation process. The TWC releases oxygen as needed during the normal reduction and oxidation process. The control module calculates the oxygen storage capacity using the difference between the pre-catalyst and post-catalyst oxygen sensor voltage levels.

Whenever the voltage levels of the post-catalyst (2) oxygen sensor nears the voltage levels that of the pre-catalyst (1) oxygen sensor, the efficiency of the catalyst is degraded.

Stepped or staged testing levels allow the control module to statistically filter the test information. This prevents falsely passing or falsely failing the oxygen storage capacity test. The calculations performed by the On-Board Diagnostic System are very complex. For this reason, do not use post catalyst oxygen sensor activity in order to determine the oxygen storage capacity unless you are directed to do this by the service information.

Three stages are used in order to monitor catalyst efficiency. Failure of the first stage indicates that the catalyst requires further testing in order to determine catalyst efficiency. Failure of the second stage indicates that the catalyst may be degraded. The third stage then looks more closely at the inputs from the pre and post O2S before determining if the catalyst is indeed degraded. This further statistical processing is done in order to increase the accuracy of the oxygen storage capacity type monitoring. Failing the first (stage 0) or the second (stage 1) test DOES NOT indicate a failed catalyst. The catalyst may be marginal or the fuel sulfur content could be very high.

Aftermarket HO2S characteristics may be different from the original equipment manufacturer sensor. This may lead to a false pass or a false fail of the catalyst monitor diagnostic. Similarly, if an aftermarket catalyst does not contain the same amount of cerium as the original part, the correlation between oxygen storage and conversion efficiency may be altered enough to set a false DTC.

Whenever a cylinder misfires, the misfire diagnostic counts the misfire and notes the crankshaft position at the time it the misfire occurred. These misfire counters are basically a file on each engine cylinder.

A current and a history misfire counter is maintained for each cylinder. The misfire current counters (Misfire Cur #1 -8) indicate the number of firing events out of the last 200 cylinder firing events which were misfires. The misfire current counters displays real time data without a misfire DTC stored. The misfire history counters (Misfire Hist #1 - 8) indicate the total number of cylinder firing events which were misfires. The misfire history counters displays 0 until the misfire diagnostic has failed and a DTC P0300 is set. Once the misfire DTC sets, the misfire history counters will be updated every 200 cylinder firing events.

The Misfire counters graphic illustrates how these misfire counters are maintained. If the misfire diagnostic reports a failure, the Diagnostic Executive reviews all of the misfire counters before reporting a DTC. This way, the Diagnostic Executive reports the most current information.

When crankshaft rotation is erratic, the control module detects a misfire condition. Because of this erratic condition, the data that is collected by the diagnostic can sometimes incorrectly identify which cylinder is misfiring. The Misfire Counters graphic shows there are misfires counted from more than one cylinder. Cylinder #1 has the majority of counted misfires. In this case, the Misfire Counters would identify cylinder #1 as the misfiring cylinder. The misfires in the other counters were just background noise caused by the erratic rotation of the crankshaft. If the number of accumulated is sufficient for the diagnostic to identify a true misfire, the diagnostic will set DTC P0300 - Misfire Detected.

Use Techline equipment to monitor misfire counter data on OBD ll compliant vehicles. Knowing which specific cylinders misfired can lead to the root cause, even when dealing with a multiple cylinder misfire. Using the information in the misfire counters, identify which cylinders are misfiring. If the counters indicate cylinders number 1 and 4 misfired, look for a circuit or component common to both cylinders number 1 and 4 such as an open ignition coil in an electronic ignition system.

Misfire counter information is located in the Specific Eng. menu, Misfire Data sub-menu of the of the data list.

The misfire diagnostic may indicate a fault due to a temporary fault not necessarily caused by a vehicle emission system malfunction. Examples include the following items:

Fuel Trim System Monitor Diagnostic Operation

This system monitors the averages of short-term and long-term fuel trim values. If these fuel trim values stay at their limits for a calibrated period of time, a malfunction is indicated. The fuel trim diagnostic compares the averages of short-term fuel trim values and long-term fuel trim values to rich and lean thresholds. If either value is within the thresholds, a pass is recorded. If either value is outside their thresholds, a rich or lean DTC will set.

In order to meet OBD ll requirements, the control module uses weighted fuel trim cells in order to determine the need to set a fuel trim DTC. A fuel trim DTC can only be set if fuel trim counts in the weighted fuel trim cells exceed specifications. This means that the vehicle could have a fuel trim problem which is causing a concern under certain conditions (i.e. the engine could be idling high due to a small vacuum leak or rough due to a large vacuum leak) while the engine operates fine at other times. No fuel trim DTC would set (although an engine idle speed DTC or HO2S DTC may set). Remember, use a scan tool in order to observe fuel trim counts while the problem is occurring.

Remember, a fuel trim DTC may be triggered by a list of vehicle faults. Make use of all information available (other DTCs stored, rich or lean condition, etc.) when diagnosing a fuel trim fault.

Comprehensive Component Monitor Diagnostic

The comprehensive component monitoring diagnostics are required to monitor emissions-related input and output powertrain components. The CARB OBD II comprehensive component monitoring list of components Intended to illuminate the malfunction indicator lamp (MIL) is a list of components, features or functions that could fall under this requirement.

Input Components

The control module monitors the input components for circuit continuity and out-of-range values. This includes performance checking. Performance checking refers to indicating a fault when the signal from a sensor does not seem reasonable, for example, a throttle position (TP) sensor that indicates high TP at low engine loads or MAP voltage. The input components may include but are not limited to the following sensors:

In addition to the circuit continuity and rationality check, the ECT sensor is monitored for the ability to achieve a steady state temperature in order to enable a closed loop fuel control.

Output Components

The output components respond to control module commands. Components where functional monitoring is not feasible will be monitored for circuit continuity and out-of-range values if applicable.

Output components to be monitored include, but are not limited to the following circuits:

California Air Resources Board (CARB) OBD II Comprehensive Component Monitoring List of Components Intended to Illuminate MIL

Important: Not all vehicles have these components:

Powertrain Control Module (PCM) Serial Data

UART Serial Data

Two methods of data transmission are used. One method involves a Universally Asynchronous Receiving/Transmitting (UART) protocol. UART is an interfacing device that allows the on board computer to send and receive serial data. Serial data refers to information which is transferred in a linear fashion - over a single line, one bit at a time. A data bus describes the electronic pathway through which serial data travels. The UART receives data in a serial format, converts the data to parallel format, and places them on the data bus (which is recognizable to the on board computer). The UART also accepts parallel data from the data bus, converts the data to serial format, and transmits them to the Tech 1 or other scan tools. This method has been the common strategy for establishing a communication link between the on board control module and the off board monitor/scanner since 1981.

Class II Serial Data

U.S. Federal regulations require that all automobile manufacturers establish a common communications system. General Motors utilizes the Class II communications system. Each bit of information can have one of two lengths: long or short. This allows vehicle wiring to be reduced by the transmission and reception of multiple signals over a single wire. The messages carried on Class II data streams are also prioritized. In other words, if two messages attempt to establish communications on the data line at the same time, only the message with higher priority will continue. The device with the lower priority message must wait. The most significant result of this regulation is that it provides scan tool manufacturers with the capability of accessing data from any make or model vehicle sold in the United States.

Data Link Connector (DLC)

The provision for communicating with the control module is the Data Link Connector (DLC). It is usually located under the instrument panel. The DLC is used to connect to a scan tool. Some common uses of the scan tool are listed below:

Powertrain Control Module (PCM) DTC Information

The Diagnostic Executive

The Diagnostic Executive is a unique segment of software which is designed to coordinate and prioritize the diagnostic procedures as well as define the protocol for recording and displaying their results. The main responsibilities of the Diagnostic Executive are:

On-Board Diagnostic Tests

A diagnostic test is a series of steps which has a beginning and end. The result of which is a pass or fail reported to the Diagnostic Executive. When a diagnostic test reports a pass result, the Diagnostic Executive records the following data:

When a diagnostic test reports a fail result, the Diagnostic Executive records the following data:

Trip

The ability for a diagnostic test to run depends largely upon whether or not a Trip has been completed. A Trip for a particular diagnostic is defined as vehicle operation, followed by an engine off period of duration and driving mode such that any particular diagnostic test has had sufficient time to complete at least once. The requirements for trips vary as they may involve items of an unrelated nature; driving style, length of trip, ambient temperature, etc. Some diagnostic tests run only once per trip (e.g. catalyst monitor) while others run continuously (e.g. misfire and fuel system monitors). If the proper enabling conditions are not met during that ignition cycle, the tests may not be complete or the test may not have run.

Warm-up Cycle

In addition, the execution of diagnostic tests may also be bound by conditions which must comprehend a Warm-up cycle. A Warm-up cycle consists of engine start-up and vehicle operation such that the coolant temperature has risen greater than a certain value (typically 40°F) from start-up temperature and reached a minimum temperature of 160°F. If this condition is not met during the ignition cycle, the diagnostic may not run.

Diagnostic Information

The diagnostic charts and functional checks are designed to locate a faulty circuit or component through a process of logical decisions. The charts are prepared with the requirement that the vehicle functioned correctly at the time of assembly and that there are not multiple faults present.

There is a continuous self-diagnosis on certain control functions. This diagnostic capability is complimented by the diagnostic procedures contained in this manual. The language of communicating the source of the malfunction is a system of diagnostic trouble codes. When a malfunction is detected by the control module, a diagnostic trouble code is set and the Malfunction Indicator Lamp (MIL) Service Engine Soon is illuminated on some applications.

Malfunction Indicator Lamp (MIL)

The MIL (Service Engine Soon) is on the instrument panel and has the following functions:

When the MIL remains ON while the engine is running, or when a malfunction is suspected due to a driveability or emissions problem, a Powertrain On-Board Diagnostic (OBD) System Check must be performed. This check will expose faults which may not be detected if other diagnostics are performed first.

MIL Requests and History Codes

(What happens when type A, B, C and D DTCs report failures and passes, and how the MIL responds. Includes criteria for turning the MIL on and off.)

The Diagnostic Executive must be able to acknowledge when all emissions related diagnostic tests have reported a pass or fail condition since the last ignition cycle. Diagnostic tests are separated into different types:

When a type A diagnostic test reports a failure, the Diagnostic Executive initiates a request to have the MIL turn on for that diagnostic test. When a type B diagnostic test reports a second failure, the Diagnostic Executive initiates a request for that diagnostic test. A type D failure records the DTC but does not illuminate the MIL. The Diagnostic Executive has the option of turning the MIL OFF when three consecutive trips are recorded where the diagnostic system passes the test. In the case of misfire or fuel trim malfunctions, there is an additional requirement that the load conditions must be within 10%, the speed conditions must be within 375 rpm, and the coolant temperatures must be in the same calibratable high or low range at the time the diagnostic test last reported a failure. For a type C diagnostic test, the Diagnostic Executive will request that the message or Auxiliary Service Lamp (if so equipped) be turned off at the next ignition key-on cycle or test passing report.

When the Diagnostic Executive requests the MIL to be turned ON, a history DTC is also recorded for the diagnostic test. To clear a history DTC for most diagnostic tests requires 40 subsequent warm-up cycles during which no diagnostic tests have reported a fail (misfire and fuel trim DTCs require 80 cycles).

Special Cases Of Type B Diagnostic Tests

Unique to the misfire diagnostic, the Diagnostic Executive has the capability of alerting the vehicle operator to potentially damaging levels of misfire. If a misfire condition exists that could potentially damage the catalytic converter as a result of high misfire levels, the Diagnostic Executive will command the MIL to flash at a rate of once per second during the time that the catalyst damaging misfire condition is present.

Fuel trim and misfire are special cases of type B diagnostics. Each time a fuel trim or misfire malfunction is detected, engine load, engine speed, and engine coolant temperatures are recorded.

When the ignition is turned off, the last reported set of conditions remain stored. During subsequent ignition cycles, the stored conditions are used as a reference for similar conditions. If a malfunction occurs during two consecutive trips, the Diagnostic Executive treats the failure as a normal type B diagnostic, and does not use the stored conditions. However, if a malfunction occurs on two non-consecutive trips, the stored conditions are compared with the current conditions. The MIL will then illuminate under the following conditions:

DTC Type Table

DTC	Description	Type	Illuminate MIL
P0101	Mass Air Flow (MAF) System Performance	A	Yes
P0102	Mass Air Flow (MAF) Sensor Circuit Low Frequency	A	Yes
P0103	Mass Air Flow (MAF) Sensor Circuit High Frequency	A	Yes
P0105	Manifold Absolute Pressure (MAP) Sensor Circuit Insufficient Activity 1997 VEHICLES ONLY	A	Yes
P0106	Manifold Absolute Pressure (MAP) System Performance	A	Yes
P0107	Manifold Absolute Pressure (MAP) Sensor Circuit Low Voltage	A	Yes
P0108	Manifold Absolute Pressure (MAP) Sensor Circuit High Voltage	A	Yes
P0111	Intake Air Temperature (IAT) Sensor Circuit Performance	D	No
P0112	Intake Air Temperature (IAT) Sensor Circuit Low Voltage	A	Yes
P0113	Intake Air Temperature (IAT) Sensor Circuit High Voltage	A	Yes
P0116	Engine Coolant Temperature (ECT) Sensor Circuit Performance	D	No
P0117	Engine Coolant Temperature (ECT) Sensor Low Voltage	A	Yes
P0118	Engine Coolant Temperature (ECT) Sensor High Voltage	A	Yes
P0120	Throttle Position (TP) System Performance	D	No
P0121	Throttle Position (TP) Sensor Circuit Insufficient Activity	A	Yes
P0122	Throttle Position (TP) Sensor Circuit Low Voltage	A	Yes
P0123	Throttle Position (TP) Sensor Circuit High Voltage	A	Yes
P0125	Engine Coolant Temperature (ECT) Excessive Time To Closed Loop Fuel Control NOT APPLICABLE TO UNLEADED EXPORT VEHICLES	A	Yes
P0131	Heated Oxygen Sensor (HO2S) Circuit Low Voltage Bank 1 Sensor 1 (Rear)	A	Yes
P0132	Heated Oxygen Sensor (HO2S) Circuit High Voltage Bank 1 Sensor 1 (Rear)	A	Yes
P0133	Heated Oxygen Sensor (HO2S) Slow Response Bank 1 Sensor 1 (Rear) NOT APPLICABLE TO UNLEADED EXPORT VEHICLES	B	Yes(2 Fails)
P0134	Heated Oxygen Sensor (HO2S) Circuit Insufficient Activity Bank 1 Sensor 1 (Rear)	A	Yes
P0135	Heated Oxygen Sensor (HO2S) Heater Circuit Bank 1 Sensor 1 (Rear)	B	Yes(2 Fails)
P0137	Heated Oxygen Sensor (HO2S) Circuit Low Voltage Bank 1 Sensor 2 (Pre-Converter) NOT APPLICABLE TO UNLEADED EXPORT VEHICLES	B	Yes(2 Fails)
P0138	Heated Oxygen Sensor (HO2S) Circuit High Voltage Bank 1 Sensor 2 (Pre-Converter) NOT APPLICABLE TO UNLEADED EXPORT VEHICLES	B	Yes(2 Fails)
P0139	Heated Oxygen Sensor (HO2S) Slow Response Bank 1 Sensor 2 (Pre-Converter) NOT APPLICABLE TO UNLEADED EXPORT VEHICLES	B	Yes(2 Fails)
P0140	Heated Oxygen Sensor (HO2S) Circuit Insufficient Activity Bank 1 Sensor 2 (Pre-Converter) NOT APPLICABLE TO UNLEADED EXPORT VEHICLES	B	Yes(2 Fails)
P0141	Heated Oxygen Sensor (HO2S) Heater Circuit Bank 1 Sensor 2 (Pre-Converter) NOT APPLICABLE TO UNLEADED EXPORT VEHICLES	B	Yes(2 Fails)
P0143	Heated Oxygen Sensor (HO2S) Circuit Low Voltage Bank 1 Sensor 3 (Post-Converter)	A	Yes
P0144	Heated Oxygen Sensor (HO2S) Circuit High Voltage Bank 1 Sensor 3 (Post-Converter)	A	Yes
P0146	Heated Oxygen Sensor (HO2S) Circuit Insufficient Activity Bank 1 Sensor 3 (Post-Converter)	B	Yes(2 Fails)
P0147	Heated Oxygen Sensor (HO2S) Heater Circuit Bank 1 Sensor 3 (Post-Converter)	B	Yes(2 Fails)
P0151	Heated Oxygen Sensor (HO2S) Circuit Low Voltage Bank 2 Sensor 1 (Front)	A	Yes
P0152	Heated Oxygen Sensor (HO2S) Circuit High Voltage Bank 2 Sensor 1 (Front)	A	Yes
P0153	Heated Oxygen Sensor (HO2S) Slow Response Bank 2 Sensor 1 (Front) NOT APPLICABLE TO UNLEADED EXPORT VEHICLES	B	Yes(2 Fails)
P0154	Heated Oxygen Sensor (HO2S) Circuit Insufficient Activity Bank 2 Sensor 1 (Front)	A	Yes
P0155	Heated Oxygen Sensor (HO2S) Heater Circuit Bank 2 Sensor 1 (Front)	B	Yes(2 Fails)
P0171	Fuel Trim System Lean Bank 1	B	Yes(2 Fails)
P0172	Fuel Trim System Rich Bank 1	B	Yes(2 Fails)
P0174	Fuel Trim System Lean Bank 2	B	Yes(2 Fails)
P0175	Fuel Trim System Rich Bank 2	B	Yes(2 Fails)
P0201	Injector #1 Control Circuit	A	Yes
P0202	Injector #2 Control Circuit	A	Yes
P0203	Injector #3 Control Circuit	A	Yes
P0204	Injector #4 Control Circuit	A	Yes
P0205	Injector #5 Control Circuit	A	Yes
P0206	Injector #6 Control Circuit	A	Yes
P0207	Injector #7 Control Circuit	A	Yes
P0208	Injector #8 Control Circuit	A	Yes
P0231	Fuel Pump Feedback Circuit Low Voltage	C	Message Only
P0232	Fuel Pump Feedback Circuit High Voltage	C	Message Only
P0300	Engine Misfire Detected	B	Yes(2 Fails)
P0322	Ignition Control (IC) Module 4x Reference Circuit No Frequency	C	Message Only
P0325	Knock Sensor Module Circuit DOMESTIC	B	Yes(2 Fails)
P0325	Knock Sensor Module Circuit UNLEADED EXPORT	D	No
P0326	Knock Sensor Circuit Excessive Spark Retard	D	No
P0327	Knock Sensor Circuit Low Voltage DOMESTIC	B	Yes(2 Fails)
P0327	Knock Sensor Circuit Low Voltage UNLEADED EXPORT	D	No
P0340	Ignition Control (IC) Module Cam Reference Circuit No Frequency	A	Yes
P0371	Ignition Control (IC) Module 24x Reference Circuit Too Many Pulses DOMESTIC	A	Yes
P0371	Ignition Control (IC) Module 24x Reference Circuit Too Many Pulses UNLEADED EXPORT	D	No
P0372	Ignition Control (IC) Module 24x Reference Circuit Missing Pulses DOMESTIC	A	Yes
P0372	Ignition Control (IC) Module 24x Reference Circuit Missing Pulses UNLEADED EXPORT	D	No
P0401	Exhaust Gas Recirculation (EGR) System DOMESTIC	A	Yes
P0401	Exhaust Gas Recirculation (EGR) System UNLEADED EXPORT	D	No
P0404	EGR System Performance 1997 VEHICLES ONLY	A	Yes
P0405	EGR Sensor Circuit Voltage Out of Range 1997 VEHICLES ONLY	A	Yes
P0420	Three Way Catalyst Low Efficiency NOT APPLICABLE TO UNLEADED EXPORT VEHICLES	A	Yes
P0441	Evaporative System No Flow During Purge	B	Yes(2 Fails)
P0506	Idle Speed Low	A	Yes
P0507	Idle Speed High	A	Yes
P0550	Power Steering Pressure (PSP) Switch Circuit Low Voltage	C	Message Only
P0560	System Voltage Low	C	Message Only
P0563	System Voltage High	C	Message Only
P0601	PCM Memory	A	Yes
P0602	PCM Not Programmed	A	Yes
P0603	PCM Memory Reset	D	No
P0606	PCM Internal Communication Interrupted	A	Yes
P1106	Manifold Absolute Pressure (MAP) Sensor Circuit Intermittent High Voltage	D	No
P1107	Manifold Absolute Pressure (MAP) Sensor Circuit Intermittent Low Voltage	D	No
P1108	BARO To MAP Signal Circuit Comparison Too High	A	Yes
P1111	Intake Air Temperature (IAT) Sensor Circuit Intermittent High Voltage	D	No
P1112	Intake Air Temperature (IAT) Sensor Circuit Intermittent Low Voltage	D	No
P1114	Engine Coolant Temperature (ECT) Sensor Circuit Intermittent Low Voltage	D	No
P1115	Engine Coolant Temperature (ECT) Sensor Circuit Intermittent High Voltage	D	No
P1121	Throttle Position (TP) Sensor Circuit Intermittent High Voltage	D	No
P1122	Throttle Position (TP) Sensor Circuit Intermittent Low Voltage	D	No
P1133	Heated Oxygen Sensor (HO2S) Insufficient Switching Bank 1 Sensor 1 (Rear) NOT APPLICABLE TO UNLEADED EXPORT VEHICLES	B	Yes(2 Fails)
P1134	Heated Oxygen Sensor (HO2S) Transition Time Ratio Bank 1 Sensor 1 (Rear) NOT APPLICABLE TO UNLEADED EXPORT VEHICLES	B	Yes(2 Fails)
P1139	Heated Oxygen Sensor (HO2S) Insufficient Switching Bank 1 Sensor 2 (Pre-Converter) NOT APPLICABLE TO UNLEADED EXPORT VEHICLES	B	Yes(2 Fails)
P1140	Heated Oxygen Sensor (HO2S) Transition Time Ratio Bank 1 Sensor 2 (Pre-Converter) NOT APPLICABLE TO UNLEADED EXPORT VEHICLES	B	Yes(2 Fails)
P1153	Heated Oxygen Sensor (HO2S) Insufficient Switching Bank 2 Sensor 1 (Front) NOT APPLICABLE TO UNLEADED EXPORT VEHICLES	B	Yes(2 Fails)
P1154	Heated Oxygen Sensor (HO2S) Transition Time Ratio Bank 2 Sensor 1 (Front) NOT APPLICABLE TO UNLEADED EXPORT VEHICLES	B	Yes(2 Fails)
P1258	Engine Metal Overtemperature Protection	A	Yes
P1320	Ignition Control (IC) Module 4x Reference Circuit Intermittent No Pulses	D	No
P1323	Ignition Control (IC) Module 24x Reference Circuit Low Frequency	A	Yes
P1350	Ignition Control System	A	Yes
P1370	IC Module 4x Reference Too Many Pulses DOMESTIC	A	Yes
P1370	IC Module 4x Reference Too Many Pulses UNLEADED EXPORT	D	No
P1371	Ignition Control (IC) Module 4x Reference Too Few Pulses DOMESTIC	A	Yes
P1371	Ignition Control (IC) Module 4x Reference Too Few Pulses UNLEADED EXPORT	D	No
P1375	Ignition Control (IC) Module 24x Reference High Voltage	A	Yes
P1376	Ignition Ground Circuit	D	No
P1377	Ignition Control (IC) Module Cam Pulse To 4x Reference Pulse Comparison	A	Yes
P1380	EBTCM DTC Detected - Rough Road Data Unusable	D	No
P1381	Misfire Detected - No EBTCM/PCM Serial Data	D	No
P1404	EGR Valve Pintle Stuck Open 1997 VEHICLES ONLY	A	Yes
P1406	Exhaust Gas Recirculation (EGR) Valve Pintle Position Circuit 1996 VEHICLES ONLY	A	Yes
P1441	Evaporative System Flow During Non-Purge	B	Yes(2 Fails)
P1508	Idle Air Control (IAC) System - Low RPM	A	Yes
P1509	IDLE Air Control (IAC) System - High RPM	A	Yes
P1520	Gear Indicator System	D	No
P1524	Throttle Position (TP) Sensor Learned Closed Throttle Angle Degrees Out-Of-Range	D	No
P1526	Throttle Position (TP) Sensor Learn Not Complete	C	Message Only
P1527	Trans Range / Pressure Switch Comparison 1997 VEHICLES ONLY	D	No
P1554	Cruise Engaged Circuit High Voltage	D	No
P1560	Cruise Control-System - Transaxle Not In Drive	D	No
P1564	Cruise Control-System - Vehicle Acceleration Too High	D	No
P1566	Cruise Control-System - Engine Rpm Too High	D	No
P1570	Cruise Control-System - Traction Control Active	D	No
P1571	Traction Control System - PWM Circuit No Frequency	D	No
P1574	EBTCM System - Stop Lamp Switch Circuit High Voltage	C	Message Only
P1575	Extended Travel Brake Switch Circuit High Voltage	C	Message Only
P1576	Brake Booster Vacuum (BBV) Sensor Circuit High Voltage	C	Message Only
P1577	Brake Booster Vacuum (BBV) Sensor Circuit Low Voltage	C	Message Only
P1578	Brake Booster Vacuum (BBV) Sensor Circuit Low Vacuum	C	Message Only
P1579	Park/Neutral To Drive/Reverse At High Throttle Angle	C	Message Only
P1599	Engine Stall Or Near Stall Detected	D	No
P1602	Loss Of EBTCM Serial Data	D	No
P1604	Loss Of IPC Serial Data	D	No
P1605	Loss Of HVAC Serial Data	D	No
P1610	Loss Of PZM Serial Data	D	No
P1611	Loss Of CVRTD Serial Data	C	Message Only
P1621	PCM Memory Performance	C	No
P1626	Theft Deterrent System - Fuel Enable Circuit	D	No
P1630	Theft Deterrent System - PCM In Learn Mode	D	No
P1631	Theft Deterrent System - Password Incorrect	D	No
P1632	Theft Deterrent System - Fuel Disabled	D	No
P1633	Ignition Supplement Power Circuit Low Voltage	A	Yes
P1634	Ignition 1 Power Circuit Low Voltage	A	Yes
P1640	Driver -1 -Input High Voltage	A	Yes
P1641	Malfunction Indicator Lamp (MIL) Control Circuit	D	No
P1642	Vehicle Speed Output Circuit	D	No
P1644	Delivered Torque Output Circuit	D	No
P1645	EVAP Solenoid Output Circuit	A	Yes
P1650	Driver -2 -Input High Voltage	A	Yes
P1652	Lift/Dive Output Circuit	D	No
P1654	Cruise Disable Output Circuit	D	No
P1660	Cooling Fan Control Circuits DOMESTIC	A	Yes
P1660	Cooling Fan Control Circuits UNLEADED EXPORT	D	No

Storing And Erasing Freeze Frame Data

Government regulations require that engine operating conditions be captured whenever the MIL is illuminated. The data captured is called freeze frame data. The freeze frame data is very similar to a single record of operating conditions. Whenever the MIL is illuminated, the corresponding record of operating conditions is recorded to the freeze frame buffer.

Each time a diagnostic test reports a failure, the current engine operating conditions are recorded in the freeze frame buffer. A subsequent failure will update the recorded operating conditions. The following operating conditions for the diagnostic test which failed typically include the following parameters:

Freeze frame data can only be overwritten with data associated with a misfire or fuel trim malfunction. Data from these faults take precedence over data associated with any other fault. The freeze frame data will not be erased unless the associated history DTC is cleared.

Intermittent Malfunction Indicator Lamp

In the case of an intermittent fault, the MIL (Check Engine) may illuminate and then (after three trips) go OFF. However, the corresponding diagnostic trouble code will be stored in the memory. When unexpected diagnostic trouble codes appear, check for an intermittent malfunction.

A diagnostic trouble code may reset. Consult the Diagnostic Aids associated with the diagnostic trouble code. A physical inspection of the applicable sub-system most often will resolve the problem.