Table 1:

Typical Drive Cycle

The powertrain control module (PCM) is designed to maintain exhaust emission levels while maintaining excellent driveability and fuel efficiency. The PCM controls the following operations:

Powertrain Control Module

The PCM is located in the engine compartment. The PCM is the control center of the vehicle.

The PCM constantly monitors the information from various sensors and controls the systems that affect vehicle performance and emissions. The PCM also performs the diagnostic functions for those systems. It can recognize operational problems and will alert the driver through the malfunction indicator lamp (MIL) when a malfunction has occurred. When the PCM detects a malfunction, it stores a diagnostic trouble code (DTC) which will help identify problem areas. This is done to aid the technician in making repairs.

The PCM supplies either 5.0 or 12.0 volts to power various sensors or switches. This is done through resistances in the PCM. The resistance is so high in value that a test lamp will not illuminate when connected to the circuit. In some cases, even an ordinary shop voltmeter will not give an accurate reading because its resistance is too low. Therefore, a DMM with a minimum of 10 megaohms input impedance is required to ensure accurate voltage readings.

The PCM controls output circuits such as the injectors, idle air control (IAC), cooling fan relays, etc. by controlling the ground or the power feed circuit through transistors or a device called an output driver module.

Torque Management

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

The PCM monitors the following sensors and engine parameters to calculate engine output torque:

The PCM monitors the engine speed in order to determine if torque reduction is required. The PCM retards the spark as appropriate to reduce engine torque output if torque reduction is required. The PCM also shuts off the fuel to certain injectors to reduce the engine power in the case of an abusive maneuver.

The following are instances when engine power reduction is likely to be experienced:

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

The PCM calculates the amount of spark retard necessary to reduce the engine power by the desired amount. The PCM disables the fuel injectors for cylinders 1, 4, 6, and 7 in the case of an abusive maneuver.

PCM Function

The PCM supplies a buffered voltage to various sensors and switches. The PCM controls most components with electronic switches which complete a ground circuit when turned ON.

Use of Circuit Testing Tools

Do not use a test lamp in order to diagnose the powertrain electrical systems unless specifically instructed by the diagnostic procedure. Use the J 35616-A connector test adapter kit, whenever diagnostic procedures call for probing any connectors.

Basic Knowledge Required

Without a basic knowledge of electricity, it will be difficult to use the diagnostic procedures contained in this section. You should understand the basic theory of 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. You should be able to read and understand a wiring diagram.

PCM Service Precautions

The PCM is designed to withstand normal current draws associated with vehicle operations. Avoid overloading any circuit. When testing for opens or shorts, do not ground any of the PCM circuits unless instructed. When testing for opens or shorts, do not apply voltage to any of the PCM circuits unless instructed. Test these circuits with a DMM only, while the PCM connectors remain connected.

Aftermarket (Add-On) Electrical And Vacuum Equipment

Aftermarket, or add-on, electrical and vacuum equipment is defined as any equipment installed on a vehicle after leaving the factory that connects to the vehicle electrical or vacuum systems. No allowances have been made in the vehicle design for this type of equipment.

Notice: Do not attach add-on vacuum operated equipment to this vehicle. The use of add-on vacuum equipment may result in damage to vehicle components or systems.

Notice: Connect any add-on electrically operated equipment to the vehicle's electrical system at the battery (power and ground) in order to prevent damage to the vehicle.

Add-on electrical equipment, even when installed to these strict guidelines, may still cause the powertrain system to malfunction. This may also include equipment not connected to the vehicle electrical system such as portable telephones and radios. Therefore, the first step in diagnosing any powertrain problem is to eliminate all aftermarket electrical equipment from the vehicle. After this is done, if the problem still exists, diagnose the problem in the normal manner.

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.

Electronic components used in the control systems are often designed in order to carry very low voltage. 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. Therefore, it is important to use care when handling and testing electronic components.

Engine Controls Information

The driveability and emissions information describes the function and operation of the PCM.

The engine controls information contains the following:

The Component System includes the following items:

The DTCs also contain diagnostic support information containing circuit diagrams, circuit or system information, and helpful diagnostic information.

Maintenance Schedule

Refer to the Shelby Maintenance Schedule of the appropriate service category for the maintenance that the owner or technician should perform in order to retain emission control performance.

Visual and Physical Underhood Inspection

Perform a careful visual and physical underhood inspection when performing any diagnostic procedure or diagnosing the cause of an emission test failure. This can often lead to repairing a problem without further steps. Use the following guidelines when performing a visual and physical inspection:

This visual and physical inspection is very important. Perform the inspection carefully and thoroughly.

Basic Knowledge Of Tools Required

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

A basic understanding of hand tools is necessary in order to effectively use this information.

System Status and Drive Cycle For Inspection/Maintenance

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

Several states require that the I/M (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 to comply with local area requirements. Using the System Status display, any of the following systems or combination of 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, diagnosis and repair is necessary in order to meet the I/M 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 Drive Cycle table to use as a guide to complete the I/M System Status tests. More than one drive cycle may be needed.

Following a DTC info clear, System Status will clear for one or all of these systems. Following a battery disconnect or a PCM replacement, all System Status information will clear.

Typical 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 Neutral, 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, fuel trim, HO2S, EVAP purge
Clutch engaged (Man), no braking, decelerate to 32 km/h (20 mph)	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, fuel trim, HO2S, EVAP purge
Decelerate, no braking. End of drive cycle	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 for the following conditions:

Diagnose the catalyst monitor heated oxygen sensors for the following functions:

Heated Oxygen Sensors (Pre-Catalyst)

The main function of the pre-catalyst heated oxygen sensor (HO2S) is to provide the PCM with exhaust stream information in order to maintain proper fueling to hold emissions within acceptable levels. These oxygen sensors are always located between the exhaust manifold and the catalytic converter. 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 PCM 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 PCM 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 would degrade oxygen sensor performance.

Catalyst Monitor Heated Oxygen Sensors

In order to control emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), the system uses a three-way catalytic converter (TWC). The catalyst 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. Catalyst monitor HO2S ( Post Catalyst HO2S) are always located downstream of the catalytic converter.

The PCM has the ability to monitor this process using the post catalyst heated oxygen sensors. The pre-sensors produce an output signal which indicates the amount of oxygen present in the exhaust gas entering the TWC. The post sensor produces an output signal which indicates the oxygen storage capacity of the catalyst. This in turn indicates the catalyst ability to convert exhaust gases efficiently. If the catalyst is operating efficiently, the pre-HO2S signal will be far more active than that produced by the post HO2S.

In addition to catalyst monitoring, the post heated oxygen sensor has a limited role in controlling fuel delivery. If the post HO2S signal indicates a high or low oxygen content for an extended period of time while in a Closed Loop, the PCM adjusts the fuel delivery slightly in order to compensate.

Catalyst Monitor Diagnostic Operation

The catalyst monitor diagnostic measures oxygen storage capacity of the catalyst converter. In order to do this, the heated sensors are installed before and after the TWC. Voltage variations between the sensors allow the PCM to determine the catalyst emission performance.

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

A good catalyst, (95 percent hydrocarbon conversion efficiency shows a relatively flat output voltage on the post-catalyst heated oxygen sensor (HO2S) signal circuit. A degraded catalyst (65 percent hydrocarbon conversion) shows a greatly increased activity in output voltage from the post catalyst HO2S.

The post-catalyst HO2S 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 the HO2S must be at operating temperature in order to achieve reliable oxygen sensor voltages like those shown in the three-way catalyst oxygen storage capacity graphic.

The PCM performs the catalyst diagnostic at idle when the conditions for running the diagnostic are met. Refer to the Conditions for Running the DTC in DTC P0420 Catalyst System Low Efficiency Bank 1 or DTC P0430 Catalyst System Low Efficiency Bank 2 . During the catalyst diagnostic, the PCM captures the current rear HO2S rich/lean status. The air fuel ratio transitions from rich to lean or lean to rich depending on the initial captured rich/lean status. The air fuel ratio transitions a second time opposite the first air fuel ratio transition. During this diagnostic, the scan tool will display HO2S voltages going from full rich to full lean. This condition is normal during this diagnostic.

The catalyst monitor diagnostic is sensitive to the following conditions:

Exhaust system leaks may cause the following:

Some of the contaminants that may be encountered are phosphorus, lead, silica, and sulfur. The presence of any of these contaminants will reduce catalyst efficiency and lead to emission failures.

Three-Way Catalyst Oxygen Storage Capacity

The PCM must monitor the three-way catalyst system (TWC) for efficiency. In order to accomplish this, the PCM 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 oxygen during its normal reduction and oxidation process. The TWC releases oxygen during its normal reduction and oxidation process. The PCM calculates the oxygen storage capacity using the difference between the pre-catalyst and post-catalyst oxygen sensor voltage levels.

Whenever the sensor activity of the post-catalyst (2) oxygen sensor nears the sensor activity of the pre-catalyst (1) oxygen sensor, the catalysts efficiency is degraded.

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 precious metal content as the original part, the correlation between oxygen storage and conversion efficiency may be altered enough to set a false DTC.

Misfire Monitor Diagnostic Operation

The misfire monitor diagnostic is based on crankshaft rotational velocity (reference period) variations. The PCM determines crankshaft rotational velocity using the crankshaft position sensor and camshaft position sensor. When a cylinder misfires the crankshaft slows down momentarily. By monitoring the crankshaft and camshaft position sensor signals, the PCM can calculate when a misfire occurs.

For a non-catalyst damaging misfire, the diagnostic is required to monitor a misfire present for between 1,000-3,200 engine revolutions.

For catalyst damage misfire, the diagnostic responds to the misfire within 200 engine revolutions.

Rough roads may cause false misfire detection. A rough road applies sudden torque variations to the drive wheels and drivetrain. This torque can intermittently decrease the crankshaft rotational velocity. The rough road sensor used for this application sends a varying voltage signal to the PCM in order to disable the misfire monitor until the rough road condition is no longer detected. Once the rough road sensor voltage has stabilized, the PCM will continue to monitor for misfire.

Whenever a cylinder misfires, the misfire diagnostic counts the misfire and notes the crankshaft position at the time the misfire occurred.

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.

When crankshaft rotation is erratic, the PCM 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 misfires is sufficient for the diagnostic to identify a true misfire, the diagnostic will set DTC P0300 - Misfire Detected. The illustration depicts an accumulation in the history buffers.

If two cylinders in sequential firing order are both misfiring, the first misfiring cylinder will accumulate misfires in its buffer but the second misfiring cylinder will not. This is because the PCM compares a misfiring cylinder with the cylinder 90 degrees prior to it in the firing order. Therefore the PCM would be comparing crankshaft speed of the second misfiring cylinder to an already suspect cylinder. The PCM however, will be able to detect both misfiring cylinders after the engine exceeds 2,000 RPM. This is because the PCM then starts to compare misfires to the opposing cylinder rather than the previous cylinder in the firing order.

Use Techline equipment to monitor the misfire counter data on applicable vehicles. Knowing which specific cylinders misfire can lead to the root cause. Using the information in the misfire counters, will 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.

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 Operation

The fuel trim 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 average of short and long-term fuel trim values. If either value is within the thresholds, a pass is recorded. If either value is outside the thresholds, a rich or lean fuel trim DTC will set.

Comprehensive Component Monitor Diagnostic

Comprehensive component monitoring diagnostics are required to monitor emissions-related input and output powertrain components.

Input Components

The PCM 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, such as a throttle position (TP) sensor that indicates high throttle position 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 its ability to achieve a steady state temperature to enable Closed Loop fuel control.

Output Components

Diagnose the output components for the proper response to PCM 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:

Wiring Harness Service

Replace the wire harnesses with the proper part number replacement. When splicing signal wires into a harness, use the wiring that has high temperature insulation.

Consider the low amperage and voltage levels utilized in the powertrain control systems. Make the best possible bond at all splices. Use rosin-core solder in these areas.

Molded-on connectors require complete replacement of the connector. Splice a new connector into the harness.

For wiring repairs and proper wiring harness conduit and tape usage, refer to Wiring Repairs in Wiring Systems.

Connectors and Terminals

In order to prevent shorting between opposite terminals, use care when probing a connector and when replacing terminals. Damage to the components could result.

Always use jumper wires between connectors for circuit checking.

Never probe through weather-pack seals.

The J 35616-A connector test adapter kit, or the equivalent, contains an assortment of flexible connectors used to probe terminals during diagnosis. Fuse remover and test tool BT-8616, or the equivalent, is used for removing a fuse and to adapt the fuse holder to a DVM for diagnosis.

Open circuits are often difficult to locate by sight because oxidation or terminal misalignment are hidden by the connectors. Merely wiggling a connector on a sensor, or in the wiring harness may temporarily correct the open circuit. Oxidized or loose connections may cause intermittent problems.

Be certain the type of connector and terminal before making any connector or terminal repair. Weather-pack and Com-pack III terminals look similar, but are serviced differently.

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. This method had been the common strategy for establishing a communication link between the on board control module and the off board monitor/scanner since 1981. UART is now used to communicate between certain modules within the vehicle.

Class 2 Serial Data

U.S. Federal regulations require that all automobile manufacturers establish a common communications system. This Shelby vehicle utilizes the Class 2 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 2 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 on either the left or the right side. The DLC is used to connect to a scan tool. Some common uses of the scan tool are listed below:

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 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 P0650 Malfunction Indicator Lamp (MIL) Control Circuit for diagnosis of service engine soon MIL.

Diagnostic Trouble Codes (DTCs)

DTCs, 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 Conditions for Running the DTC. When these conditions exist, the PCM checks certain circuits or systems for a malfunction. These checks are called Conditions for Setting the DTC. When the setting conditions are true, a malfunction is indicated and the DTC is stored as failed. Some DTCs alert the driver through the SERVICE ENGINE SOON MIL or a message. Other DTCs don't trigger a driver warning. Refer to the Diagnostic Trouble Code (DTC) List table for a complete list powertrain control module DTCs and what driver alerts they trigger. The PCM also saves data and input parameters when most DTCs are set. The saved information is called Freeze Frame and Failure Records.

Trip

The ability for a DTC to run depends largely upon whether or not a trip has been completed. A trip for a particular DTC is defined as vehicle operation, followed by an engine off period and a driving mode such that any particular DTC has had sufficient time to complete testing. The requirements for trips vary as they may involve items of an unrelated nature such as driving style, length of trip, ambient temperature, etc. Some DTCs 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 a DTC 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 more 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 DTC may not run.

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 as Freeze Frame data. A subsequent failure will not update the recorded operating conditions.

The Freeze Frame data parameters stored with a DTC failure include the following:

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.

Failure Records Data

In addition to Freeze Frame data, the PCM may also store Failure Records data when a DTC reports a failure. Unlike Freeze Frame data, Failure Records data can be stored by DTCs that DO NOT illuminate the MIL.

Failure Records save many more relevant data parameters than Freeze Frame records. Which data parameters are saved varies based on the DTC set. Transaxle DTC Failure Records generally contain transaxle related data parameters while fuel DTC Failure Records generally contain fuel related data parameters. If several DTCs are set, Failure Records for the three most recently set DTCs will be saved.

Freeze Frame, Failure Records data may be retrieved through the Diagnostic Trouble Code 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.

Important: Always capture the Freeze Frame and Failure Records information with the scan tool BEFORE proceeding with diagnosis. Clearing DTCs, disconnecting the battery, disconnecting PCM or body connectors or procedures performed during diagnosis may ERASE or overwrite the stored Freeze Frame and Failure Records data. Loss of this data may prevent accurate diagnosis of an intermittent or difficult to set DTC.

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.

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.

DTC Types

DTCs are categorized by type. The type will indicate the action the DTC will take in storing a failure and illuminating the MIL. The following table indicates what action each DTC type will take when a failure is recorded. Type C DTCs that DO NOT display a message were formerly referred to as Type D.

DTC Type	MIL Illumination	Freeze Frame Stored	Failure Records Stored
A	Yes	Yes	Yes
B	Yes (with two fails)	Yes (on first fail)	Yes (on second fail)
C	NO A Message may display	NO	Varies

In order for a type B DTC to request MIL illumination the DTC must fail in two consecutive drive trips in which the DTC tests.

Refer to the Diagnostic Trouble Code (DTC) List table for the type of each powertrain control module DTC.

Always refer to the diagnostic support information within each DTC to obtain the Action Taken when the DTC Sets and the Conditions for Clearing the DTC. These will indicate any variations from the general type A, B, and C actions.

Special Cases Of Type B Diagnostic Tests

Fuel trim and misfire are special cases of type B DTCs. Each time a fuel trim or misfire malfunction is detected, engine load, engine speed, and engine coolant temperatures are recorded. In order for the fuel trim or misfire DTCs to report a PASS the load conditions must be within 10 percent, 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

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:

Unique to the misfire diagnostic, the misfire DTC has the ability 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 PCM will command the MIL to flash at a rate of once per second during the time that the catalyst damaging misfire condition is present.

PCM Snapshot Using A Scan Tool

PCM snapshot data may also be taken and retrieved using a diagnostic tool by selecting the Snapshot option. These parameters can then be reviewed at any time.

Keep in mind that once a 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.

Diagnostic Trouble Code Display

DTCs can only be displayed with the use of a scan tool.

Scan Tool Parameter - DTC Status

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

Scan Tool Parameter - Fail This Ignition

This selection will display all DTCs that have failed during the present ignition cycle.

Scan Tool Parameter - Last Test Fail

This selection will display only DTCs 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 DTCs, the last failure must have occurred during the current ignition cycle to appear as Last Test Fail.

Scan Tool Parameter - MIL SVS or Message Requested

This selection will display only DTCs that are requesting the MIL or a message be displayed. Type C DTCs that do not request a message cannot be displayed using this option. This selection will report type B DTCs only after they have failed twice and requested the MIL.

Scan Tool Parameter - Not Run Since Code Clear

This selection will display DTCs that have not tested since codes were last cleared. Since any displayed DTCs have not run, their condition (passing or failing) is unknown.

Scan Tool Parameter - Test Fail Since Code Clear

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

Scan Tool Parameter - History Code

This selection will display only DTCs that are stored to the PCMs history memory. It will not display type B DTCs that have not requested the MIL. It will display all type A DTCs, and type B DTCs which have requested the MIL, that have failed within the last 40 warm-up cycles. In addition, it will display all type C DTCs that have failed within the last 40 warm-up cycles. Fuel trim and misfire DTCs are stored for 80 warm-up cycles.

Clearing Diagnostic Trouble Codes

Use a scan tool to clear DTCs from the PCM memory. Disconnecting the vehicle battery will also clear the PCM memory, however most other system memories will also be cleared. This is generally undesirable. Disconnecting the PCM soley for clearing DTCs is not recommended since this unnecessarily disturbs the connections. Before clearing DTCs the scan tool has the capability to save any data stored with the DTCs 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 DTCs so that any future service work is not needlessly confused.

Many DTCs have complex running and setting conditions. Therefore, simply clearing DTCs 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.