Vehicle Control Module Serial Data Communication

Class II Serial Data

U.S. Federal regulations require that all automobile manufacturers establish a common communications system. This system 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 the regulation provides scan tool manufacturers with the capability of accessing data from any make or model vehicle sold in the United States.

The Diagnostic Executive

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

On-Board Diagnostic Tests

A diagnostic test is a series of steps which has a beginning and an 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 a key ON and key OFF cycle in which all the enabling criteria for a given diagnostic have been met allowing the diagnostic to run. 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 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

A warm-up cycle consists of an engine start-up and vehicle operation such that the coolant temperature has risen more than 4°C (40°F) from the start-up temperature and reached a minimum engine coolant temperature of 71°C (160°F). If this condition is not met during the ignition cycle, the diagnostic may not run.

Diagnostic Information

The diagnostic tables and the functional checks are designed in order to locate a poor circuit or a malfunctioning component through a process of logical decisions. The tables are prepared with the assumption that the vehicle functioned correctly at the time of assembly and that there are no multiple faults present.

There is a continuous self-diagnosis on certain control functions. This diagnostic capability is complemented by the diagnostic procedures which are 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 will set and the malfunction indicator lamp (MIL) will illuminate on some applications.

Malfunction Indicator Lamp (MIL)

The malfunction indicator lamp (MIL) is on the instrument panel. The MIL 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, perform an On-Board Diagnostic (OBD) System Check. The procedures for these checks are given in engine controls. These checks expose faults which the technician may not detect if other diagnostics are performed first.

MIL Requests and History Codes

The diagnostic executive must acknowledge when all the emissions related diagnostic tests have reported a pass or fail condition since the last ignition cycle. Each diagnostic test is separated into 4 types:

When a type A diagnostic test reports a failure, the diagnostic executive immediately requests to have the MIL turn ON for that diagnostic test. When a type B diagnostic test reports a failure during 2 consecutive trips, the diagnostic executive turns on the MIL for that diagnostic test. The diagnostic executive has the option of turning the MIL OFF when the diagnostic test which caused the MIL to illuminate the passes for 3 consecutive trips. In the case of misfire or fuel trim malfunctions, there are additional requirements as follows:

When the diagnostic executive requests the service light to be turned ON or a type C diagnostic fault is reported, a history DTC is also recorded for the diagnostic test. The provision for clearing a history DTC for any diagnostic tests requires 40 subsequent warm-up cycles during which no diagnostic tests have reported a fail, a battery disconnect, or a scan tool clear info command.

Special Cases Of Type A Diagnostic Tests

Unique to the misfire diagnostic, the diagnostic executive has the capability of alerting the driver of potentially damaging levels of misfire. If a misfire condition exists that could potentially damage the catalytic converter, the diagnostic executive will command the MIL to flash at a rate of once per second during the times that the catalyst damaging misfire condition is present.

Special Cases Of Type B Diagnostic Tests

Misfire and fuel trim malfunctions are special cases of type B diagnostics. Each time a fuel trim malfunction is detected, the engine load, the engine speed, and the 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 fuel trim malfunction occurs during 2 consecutive trips, the diagnostic executive treats the failure as a normal type B diagnostic. The diagnostic executive does not use the stored conditions. However, if a fuel trim malfunction occurs on 2 non-consecutive trips, the stored conditions are compared with the current conditions. The MIL will then illuminate under the following conditions:

Storing And Erasing Freeze Frame Data

Government regulations require that the engine operating conditions are to be captured whenever the MIL is illuminated. The data that is 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 failure records 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 the data associated with a misfire or a fuel trim malfunction. The data from these faults take precedence over data that is 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 malfunction indicator lamp (MIL) may illuminate and then after 3 trips turn OFF. However, the corresponding diagnostic trouble code will store in the memory. When unexpected diagnostic trouble codes appear, check for an intermittent malfunction.

Data Link Connector (DLC)

The provision for communicating with the control module is a data link connector (DLC). The DLC is usually located under the instrument panel. The DLC is used in order to connect to a scan tool. Some common uses of the scan tool are listed below:

Control Module Learning Ability

The control module has a learning ability which allows the control module to make corrections for minor variations in the fuel system in order to improve driveability. Whenever the battery cable is disconnected, the learning process resets.

The driver may note a change in vehicle performance. In order to allow the PCM to re-learn to drive the vehicle at part throttle with moderate acceleration. The vehicle may also operate at idle conditions until the normal performance returns.

Reprogramming (Flashing) The Control Module

Some vehicles allow the reprogramming of the control module without removal from the vehicle. This provides a flexible and a cost-effective method of making changes in software and calibrations.

Refer to the latest Techline information on reprogramming or flashing procedures.

Verifying Vehicle Repair

Verification of the vehicle repair will be more comprehensive for vehicles with OBD II system diagnostics. Following a repair, the technician should perform the following steps:

1. Review the fail records and the Freeze Fame data for the DTC which was diagnosed. Record the fail records or Freeze Fame data. The Freeze Frame data will only store for an A or B type diagnostic.
2. Clear the DTCs.
3. Operate the vehicle within the conditions noted in the fail records or the Freeze Frame data.
4. Monitor the DTC status information for the specific DTC which has been diagnosed until the diagnostic test associated with that DTC runs.

Following these steps are very important in verifying repairs on the OBD II systems. Failure to follow these steps could result in an unnecessary repair.

Reading Diagnostic Trouble Codes

Use a diagnostic scan tool in order to read the diagnostic trouble codes. Failure to follow this step could result in unnecessary repairs.

Clearing Diagnostic Trouble Codes

In order to clear Diagnostic Trouble Codes (DTCs), use the diagnostic scan tool clear DTCs or clear info function. When clearing DTCs follow the instructions supplied by the tool manufacturer. When a scan tool is not available, disconnecting one of the following sources for at least thirty (30) seconds can also clear the DTCs:

Notice: Turn off the ignition key when disconnecting or reconnecting battery power in order to prevent system damage.

Disconnecting the negative battery cable may result in the loss of other on-board memory data, such as preset radio tuning.

DTC Modes

The OBD II vehicles have three options available in the scan tool DTC mode in order to display the enhanced information available. A description of the new modes, the DTC Info and the Specific DTC, follows. After selecting the DTC, the following menu appears:

The following is a brief description of each of the sub menus in the DTC Info and the Specific DTC. The order in which they appear here is alphabetical and not necessarily the way they will appear on the scan tool.

DTC Info Mode

Use the DTC Info mode in order to search for a specific type of stored DTC information. There are seven choices. The electronic service information may instruct the technician to test for DTCs in a certain manner. Always follow the published service procedures.

In order to get a complete description of any status, press the Enter key before pressing the desired F-key. For example, pressing enter, then an F key will display a definition of the abbreviated scan tool status.

DTC Status

This selection displays any DTCs that have not run during the current ignition cycle or have reported a test failure during this ignition up to a maximum of 33 DTCs. The DTC tests which run and pass removes that DTC number from the scan tool screen.

Fail This Ign. (Fail This Ignition)

This selection displays all of the DTCs that have failed during the present ignition cycle.

History

This selection displays only the DTCs that are stored to the history memory of the control module. The history memory will not display the Type B DTCs that have not requested the MIL. The history memory will display all of the type A and B DTCs that have requested the MIL and have failed within the last 40 warm-up cycles. In addition, the history memory will display all of the type C DTCs that have failed within the last 40 warm-up cycles.

Last Test Fail

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

MIL Request

This selection displays only the DTCs that are requesting the MIL. Type C DTCs cannot be displayed by using this option. This selection will report type B DTCs only after the MIL has been requested.

Not Run SCC (Not Run Since Code Clear)

This option displays up to 33 DTCs that have not run since the DTCs were last cleared. Since any displayed DTCs have not run, their condition, passing or failing, is unknown.

Test Fail SCC (Test Failed Since Code Clear)

This selection displays all of the active and history DTCs that have reported a test failure since the last time the DTCs were cleared. The DTCs that last failed over 40 warm-up cycles (before this option is selected) will not be displayed.

Specific DTC Mode

This mode is used in order to check the status of the individual diagnostic tests by the DTC number. This selection can be accessed if a DTC has passed or failed. Many OBD II DTC mode descriptions are possible because of the extensive amount of information that the Diagnostic Executive monitors regarding each test. Some of the many possible descriptions follow with a brief explanation.

This selection only allows the entry of the DTC numbers that are supported by the vehicle that is being tested. If an attempt is made to enter the DTC numbers for tests which the diagnostic executive does not recognize, the requested information will not be displayed correctly and the scan tool may display an error message. The same applies to using the DTC trigger option in the snapshot mode. If an invalid DTC is entered, the scan tool will not trigger.

Failed Last Test

For type A and B DTCs, this message will display during the subsequent ignition cycles until the test passes or the DTCs are cleared. For type C DTCs, this message clears whenever the ignition is cycled.

Failed Since Clear

This message displayed indicates that the diagnostic test failed at least once within the last 40 warm-up cycles since the last time the control module cleared the DTCs.

Failed This Ign. (Failed This Ignition)

This message displayed indicates that the diagnostic test has failed at least once during the current ignition cycle. This message will clear when the DTCs are cleared or the ignition is cycled.

History DTC

This message displayed indicates that the DTC has stored to memory as a valid fault. A DTC displayed as a history fault does not necessarily mean that the fault is no longer present. The history description means that all of the conditions necessary for reporting a fault have met.

MIL Requested

This message displayed indicates that the DTC is currently causing the MIL to turn ON. Remember that only type A and B DTCs can request the MIL. The MIL request cannot determine if the DTC fault conditions are currently being experienced. This is because the diagnostic executive requires up to 3 trips during which the diagnostic test passes to turn OFF the MIL.

Not Run Since Cl (Not Run Since Cleared)

This message displayed indicates that the selected diagnostic test has not run since the last time the DTCs were cleared. Therefore, the diagnostic test status, passing or failing, is unknown. After the DTCs are cleared, this message continues to be displayed until the diagnostic test runs.

Not Run This Ign. (Not Run This Ignition)

This message displayed indicates that the selected diagnostic test has not run this ignition cycle.

Test Ran and Passed

This message displayed indicates that the selected diagnostic test has the following items:

Whenever the indicated status of the vehicle is Test Ran and Passed after a repair verification, the vehicle is ready to be released to the customer.

If the indicated status of the vehicle is Failed This Ign after a repair verification, then the repair is incomplete. A further diagnosis is required.

Prior to repairing a vehicle, use the status information in order to evaluate the state of the diagnostic test and to help identify an intermittent problem. The technician can conclude that although the MIL is illuminated, the fault condition that caused the code to set is not present. An intermittent condition must be the cause.

Vehicle Control Module General Description

The VCM is located on the right hand side fenderwell. The VCM is the control center for the emissions, the powertrain and the antilock brakes systems.

The VCM constantly monitors the information from the various sensors. The VCM controls the component systems which affect the engine, the transmission, and the antilock operation.

The VCM alerts the driver through the Malfunction Indicator Lamp (MIL) or the antilock indicator lamp. The VCM stores the DTCs which identify the problem areas for the technician making repairs. Refer to Vehicle Control Module General Description , for further information on using the diagnostic function of the VCM for engine operation.

Refer to Transmission for the diagnosis of the automatic transmission. Refer to Section 5E3B in the appropriate service manual for further information on the antilock brakes.

The VCM in this vehicle is programmable. The only services allowed on the VCM is the control module replacement with the KS calibrator PROM transferred or the KS calibrator PROM only.

The KS calibrator contains the up integrated knock sensor calibration. The VCM stores the 4 calibrations in the Electronically Erasable Programmable Read Only Memory (EEPROM).

When replacing the VCM, programming the EEPROM and transferring the KS calibrator PROM to the new VCM is mandatory. Refer to EEPROM Programming .

The 4 calibrations required for the VCM are the Powertrain, the ABS, the VSS buffer, and the A/C. Each calibration has its own part number. Determine the correct calibrations for a particular vehicle based on the VIN number of the vehicle.

Vehicle Control Module (VCM)

The VCM processes the various input information. The VCM sends the necessary electrical responses to the control fuel delivery, the spark timing, and the other emission control systems. The input information interrelates to more than one output; therefore, if the one input fails, the failure can affect more than one system.

VCM Learning Ability

Notice: Since the VCM is located under the hood, its connectors are sealed and cannot be backprobed as in the previous model years. Do not attempt to backprobe as a connector or seal damage could occur.

The learning ability of the VCM allows the VCM to make corrections for minor variations in the fuel system in order to improve driveability.

When the battery is disconnected for other repairs, the learned connections are lost. The driver may note a change in the performance of the vehicle. The engine must be at normal operating temperature for learning to occur. Drive the vehicle at part throttle with a moderate acceleration and idle condition until the normal performance returns.

Vehicle Control Module VCM

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.

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 procedures. Use the J 35616-A Connector Test Adapter Kit whenever the diagnostic procedures call for probing any of the connectors.

Control Module Service Precautions

The control module is designed to withstand the normal current draws that are associated with the vehicle operations. Avoid overloading any circuit. When testing for opens or shorts, do not ground any of the control module circuits unless instructed. When testing for opens or shorts, do not apply voltage to any of the control module circuits unless instructed. Only test these circuits with DMM J 39200 , while the control module connectors remain connected to the control module.

Aftermarket (Add-On) Electrical And Vacuum Equipment

The aftermarket (add-on) electrical and vacuum equipment is defined as any equipment that is installed on a vehicle after leaving the factory that connects to the electrical or vacuum systems of the vehicle. 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.

The add-on electrical equipment, even when installed to these strict guidelines, may still cause the powertrain system to malfunction. This may also include any equipment which is not connected to the electrical system of the vehicle such as portable telephones and radios. Therefore, the first step in diagnosing any powertrain problem is to remove all of the aftermarket electrical connections 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.

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.

Engine Controls Information

The driveability and emissions information describes the function and operation of the control module. The emphasis is placed on the diagnosis and repair of problems related to the system.

Engine components, wiring diagrams, and diagnostic tables:

The Component System includes the following items:

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

Maintenance Schedule

Refer to the Maintenance Schedule in Section 0B of the appropriate service manual 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

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.

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

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) are 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. Refer to Heated Oxygen Sensor (HO2S) Replacement for the 2.2L or to Heated Oxygen Sensor (HO2S) Replacement for the 4.3L.

HO2S Heater

The heated oxygen sensors are used in order to minimize the amount of time required for a closed loop fuel control operation and in order to allow for an accurate catalyst monitoring. The oxygen sensor heater greatly decreases the amount of time required for the fuel control sensors Bank 1 HO2S 1 and Bank 2 HO2S 1 to become active. The oxygen sensor heaters are required by catalyst monitor sensors Bank 1 HO2S 2 and Bank 1 HO2S 3 in order to maintain a sufficiently high temperature which allows accurate exhaust oxygen content readings that are further from the engine.

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 ability of the catalyst to convert the 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). The 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 capacity of the catalyst to store and release oxygen generally degrades. The OBD II catalyst monitor diagnostic is based on a correlation between conversion efficiency and oxygen storage capacity.

A good catalyst, which operates at approximately 95 percent hydrocarbon conversion efficiency, shows a relatively flat output voltage on the post-catalyst heated oxygen sensor (HO2S). A degraded catalyst, which may operate at approximately 65 percent 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, the HO2S 2, and the HO2S 3 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 can lead to 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.

Evaporative Emission (EVAP) Purge System Vacuum Switch