GM Service Manual Online
For 1990-2009 cars only

Object Number: 848016  Size: SH

The powertrain control module (PCM) is a precision 32-bit microprocessor and is an essential part of the electronic control system. The PCM is responsible for such major functions as control of the fuel injectors, the idle air control (IAC) valve, the fuel pump relay, etc. The PCM (1) is located below the instrument panel (IP) storage compartment. Communication with the PCM is through the data link connector (2) located on the left side below the IP.

The PCM performs the OBD II diagnostic tests of the emission related systems. The PCM supplies a buffered voltage, called reference voltage, to the various information sensors and switches. The PCM controls most components with an electronic switch that completes a ground circuit when turned ON. The electronic switch is commonly referred to as an output driver. The PCM is also responsible for a self-diagnosis function and a fail-safe function.

Self-Diagnosis Function

The powertrain control module (PCM) diagnoses any troubles which may occur in the engine control system when the ignition switch is in the ON position with the engine running. The PCM indicates a malfunction by illuminating the malfunction indicator lamp (MIL) when a fault occurs in any of the following systems:

    • The heated oxygen sensor 1 (HO2S 1)
    • The heated oxygen sensor 2 (HO2S 2)
    • The engine coolant temperature (ECT) sensor
    • The accelerator pedal position (APP) sensors
    • The throttle position (TP) sensors
    • The vehicle speed sensor (VSS)
    • The intake air temperature (IAT) sensor
    • The mass air flow (MAF) sensor
    • The camshaft position (CMP) sensor
    • The crankshaft position (CKP) sensor
    • The knock sensor (KS) system
    • The evaporative emission (EVAP) control system
    • The idle air control (IAC) system
    • The electronic throttle control (ETC) or throttle actuator control (TAC) system
    • The CMP actuator solenoid system
    • The misfire detection
    • The fuel-trim
    • The catalyst monitor
    • The central processing unit (CPU) of the PCM

When the PCM detects a malfunction in one of the above areas, the PCM will illuminate or flash the MIL in order to notify the driver of the occurrence of a fault.

Malfunction Indicator Lamp (MIL) Operation

The malfunction indicator lamp (MIL) is located in the instrument panel cluster. The MIL will display as either SERVICE ENGINE SOON or one of the following symbols when commanded ON:


Object Number: 800893  Size: SH

Object Number: 802451  Size: SH

The MIL indicates that an emissions related fault has occurred and vehicle service is required.

The following is a list of the modes of operation for the MIL:

    • The MIL illuminates when the ignition is turned ON, with the engine OFF. This is a bulb test to ensure the MIL is able to illuminate.
    • The MIL turns OFF after the engine is started if a diagnostic fault is not present.
    • The MIL remains illuminated after the engine is started if the control module detects a fault. A diagnostic trouble code (DTC) is stored any time the control module illuminates the MIL due to an emissions related fault. The MIL turns OFF after three consecutive ignition cycles in which a Test Passed has been reported for the diagnostic test that originally caused the MIL to illuminate.
    • The MIL flashes if the control module detects a misfire condition which could damage the catalytic converter.
    • When the MIL is illuminated and the engine stalls, the MIL will remain illuminated as long as the ignition is ON.
    • When the MIL is not illuminated and the engine stalls, the MIL will not illuminate until the ignition is cycled OFF and then ON.

Fail-Safe Function

When a malfunction occurs within the engine control system, the PCM maintains control over the fuel injection system, the idle speed control system, etc. The PCM controls these systems by using calculated values and/or backup programs stored within the PCM.

This function is called the fail-safe function. With the fail-safe function, a certain level of engine performance is available even when a malfunction occurs. The fail-safe function prevents a complete loss of engine performance.

The systems covered by the fail-safe function are as follows:

    • The MAF sensor
    • The IAT sensor
    • The APP sensor
    • The TP sensor
    • The HO2S heater circuits
    • The KS system
    • The TAC system
    • The CPU in the PCM
    • The fuel cut-off for ignition system failures

Control Module Learning Ability

The powertrain control module has a "learning" ability which enables the control module to make corrections for minor variations in the fuel system. This learning ability can improve driveability. Disconnecting the battery resets the learning process. A change in the vehicle's performance may be noticed when a reset from a PCM power down occurs. Operating the vehicle under varying conditions will enable the control module to regain any lost vehicle performance.

In order to initiate the control module learning ability, warm the engine to operating temperature and drive the vehicle at part throttle with moderate acceleration. Continue to drive the vehicle while including steady cruise and idle speed operation. For the best idle speed quality several key cycles with a short drive and long idle periods is recommended.

Check Mode

The PCM has the ability to operate in the Normal mode or in the Check Mode.The 2 modes of operation are similar. When the PCM is operating in the Check mode, the PCM has an increased ability to detect malfunctions. The most significant feature of the Check mode operation is that all B type codes now run like A type codes. In order to request that the PCM operate in the Check Mode, scan tool communication with the PCM is necessary. For information on the use of the Check Mode in diagnosing driveability concerns, refer to Service Bay Test.

PCM Output Controls

The powertrain control module (PCM) can be directed by a scan tool to operate certain solenoids, valves, motors, and switches. This scan tool function is generally referred to as Output Controls. The Output Controls can be found under Special Functions selection of the scan tool. Some Output Controls may be disabled by the PCM during certain types of vehicle operation. Operating a PCM controlled device with the scan tool should be limited to a maximum of ten seconds per test period.

Data Link Connector (DLC)


Object Number: 828218  Size: SH

Important: Do not use a scan tool that displays faulty data. Report the scan tool problem to the manufacturer. Use of a faulty scan tool can result in misdiagnosis and unnecessary parts replacement.

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

    • Identifying stored diagnostic trouble codes (DTCs)
    • Clearing the DTCs
    • Performing output control tests
    • Reading the serial data

Reading Diagnostic Trouble Codes

The procedure for reading diagnostic trouble codes is to use a diagnostic scan tool. Follow the instructions supplied by the scan tool manufacturer in order to read DTCs accurately.

Clearing Diagnostic Trouble Codes

Important: Do not clear the DTCs unless directed to do so by the service information provided for each diagnostic procedure. The Freeze Frame data which may help diagnose an intermittent fault will be erased from the memory when the DTCs are cleared.

The PCM will begin to count the warm-up cycles when the fault that caused the DTC to be stored into memory has been corrected. The DTC will automatically be cleared from the PCM memory when the PCM has counted 40 consecutive warm-up cycles with no further faults detected.

Diagnostic trouble codes (DTCs) can be cleared using a scan tool. In order to clear DTCs, use the scan tools Clear DTC Information function. Follow the instructions supplied by the scan tool manufacturer.

Aftermarket (Add-On) Electrical and Vacuum Equipment

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.

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.

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

Add-on electrical equipment may cause the engine control system to malfunction even when the add-on electrical equipment is installed properly. Portable telephones and radios may also cause engine control system malfunctions even when not connected to the vehicles electrical system. The first step in diagnosing any engine control system problem is to remove all aftermarket electrical equipment from the vehicle. Diagnosis may proceed in the normal manner after eliminating aftermarket equipment as a cause of the engine control system malfunction.

Electrostatic Discharge (ESD) 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 engine control system are often designed to operate at very low voltages. Electronic components are susceptible to damage caused by electrostatic discharge. Less than 100 volts of static electricity can cause damage to some of the 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. Therefore, use care when handling and testing electronic components in order to avoid electrostatic charges that can cause electronic component damage.

Input Components

The PCM supplies a buffered (reference) voltage to the various information sensors and switches. The PCM monitors the input components for circuit continuity and out-of-range values. The PCM also provides performance checking. Performance checking refers to the PCM indicating a fault when the signal from an input does not seem reasonable, i.e., a throttle position (TP) sensor that indicates a high throttle position at low engine loads or low manifold absolute pressure sensor voltage. The input components may include, but are not limited to the following sensors and switches:

    • The crankshaft position (CKP) sensor
    • The accelerator pedal position (APP) sensors
    • The throttle position (TP) sensors
    • The engine coolant temperature (ECT) sensor
    • The camshaft position (CMP) sensor
    • The mass air flow (MAF) sensor
    • The heated oxygen sensor (HO2S)
    • The fuel tank pressure (FTP) sensor
    • The knock sensor (KS)
    • The vehicle speed sensor (VSS)
    • The power steering pressure (PSP) switch, if equipped
    • The transmission range switch (A/T only)
    • The A/C compressor control module (A/C relay)

Output Components

The PCM is responsible for the control and operation of many output components. The PCM controls many components with an electronic switch called an output driver that completes a ground circuit when turned ON. The PCM monitors the output components for the proper response to the 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:

    • The idle air control (IAC) valve
    • The CMP actuator solenoid valve
    • The circuit opening or fuel pump relay
    • The TAC motor
    • The EVAP system solenoids
    • The EVAP vacuum leak pump
    • The malfunction indicator lamp (MIL) control
    • The A/C compressor control module (A/C relay)
    • The electronic transaxle controls

Catalyst Monitor Diagnostic Operation

The powertrain control module (PCM) uses certain diagnostic strategies known as primary system based diagnostics that evaluate the various primary system operations. The primary system based diagnostics also evaluate the various primary system operations affect on vehicle emissions. Some of the primary system based diagnostics are listed here with a brief functional description of the diagnostics involved.

The OBD 2 catalyst monitor diagnostic measures the oxygen storage capacity of the 3-way catalytic converter (TWC). Heated oxygen sensors (HO2S) are installed before (pre-catalyst) and after (post-catalyst) the TWC. Voltage variations between the sensors allow the PCM to determine the performance of the TWC catalyst. When the TWC catalyst becomes less effective in promoting chemical reactions, the catalyst's capacity to store and release oxygen is generally degraded. The OBD 2 catalyst monitor diagnostic is based on a correlation between the conversion efficiency of the TWC catalyst and the oxygen storage capacity of the catalyst. A good catalyst, e.g., 95 percent hydrocarbon conversion efficiency, will show a relatively flat output voltage on the post-catalyst sensor, HO2S 2. A degraded catalyst, 65 percent hydrocarbon conversion, will show greatly increased activity in the output voltage from the post catalyst HO2S.

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

The catalyst monitor diagnostic is sensitive to the following conditions:

    • Exhaust leaks
    • HO2S contamination
    • Alternative fuels

Exhaust system leaks may cause any of the following results:

    • A false failure for a normally functioning, good catalyst.
    • Prevent a degraded catalyst from failing the catalyst monitor diagnostic.
    • Prevent the catalyst monitor diagnostic from running.

The presence of HO2S contaminants may prevent the catalyst monitor diagnostic from functioning properly.

Three-Way Catalyst Oxygen Storage Capacity

The TWC catalyst must be monitored for efficiency. In order to accomplish this, the control module monitors the pre-catalyst (HO2S 1) and post-catalyst (HO2S 2) oxygen sensors. When the TWC is operating properly, the post-catalyst oxygen sensor will have significantly less activity than the pre-catalyst oxygen sensor. The TWC stores and releases oxygen as needed during the normal reduction and oxidation process. The control module will calculate the oxygen storage capacity using the difference between the pre-catalyst and post-catalyst oxygen sensor voltage levels. If the activity of the post-catalyst oxygen sensor approaches that of the pre-catalyst oxygen sensor, the catalyst's efficiency is degraded.

Stepped or staged testing levels allow the PCM to statistically filter test information. This prevents falsely passing or falsely failing the catalyst monitor oxygen storage capacity test. The calculations performed by the on-board diagnostic system are very complex. Post-catalyst oxygen sensor activity should not be used to determine the oxygen storage capacity unless directed by the service manual.

A 2-stage test is used to monitor the catalyst efficiency. Failure of the first stage of the test will indicate that the catalyst requires further testing in order to determine the catalyst efficiency. The second stage test looks at the inputs from the pre-catalyst and post-catalyst HO2S sensors more closely in order to determine if the catalyst is actually degraded. This two stage test further increases the accuracy of the oxygen storage capacity monitor. Failing the first stage test DOES NOT indicate a failed catalyst. The catalyst may be marginal or the fuels sulfur content could be very high.

Aftermarket HO2S characteristics may be significantly different from the original equipment manufacturer HO2S. An inferior HO2S may lead to a false pass or a false fail of the catalyst monitor diagnostic. An aftermarket catalytic converter that does not contain the same amount of cerium as the original catalytic converter can cause a false DTC to set. An incorrect amount of cerium in the catalyst can alter the correlation between the oxygen storage and the conversion efficiency of the TWC.

Catalyst Monitor (Good Catalyst)


Object Number: 14442  Size: SH

A good TWC catalyst will show a very active output voltage on the pre-catalyst heated oxygen sensor (1). A good catalyst, 95 percent hydrocarbon conversion, will show a relatively flat output voltage on the post-catalyst heated oxygen sensor (2).

Catalyst Monitor (Bad Catalyst)


Object Number: 14446  Size: SH

A degraded TWC catalyst, 65 percent hydrocarbon conversion, will show greatly increased activity in the output voltage from the post-catalyst heated oxygen sensor (2). The degraded catalyst post-catalyst HO2S output voltage will therefore appear similar to the typically active output voltage of the pre-catalyst heated oxygen sensor (1).

Misfire Monitor Diagnostic Operation

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

For a non-catalyst damaging misfire, the diagnostic will be required to report a misfire that is present within 1000-3200 engine revolutions.

For a catalyst damaging misfire, the diagnostic will respond to a misfire that is within 200 engine revolutions.

Rough roads may cause a false misfire detection. A rough road will cause torque to be applied to the drive wheels and the drive train. This torque can intermittently decrease the crankshaft rotational velocity and cause a false misfire detection.

On automatic transaxle equipped vehicles, the torque converter clutch (TCC) will be disabled whenever a misfire is detected. Disabling the TCC isolates the engine from the rest of the drive line and minimizes the effect of the drive wheel inputs (torque) on the crankshaft rotation.

When the TCC has been disabled as a result of a misfire detection, the TCC will be re-enabled after approximately 3200 engine revolutions with no misfire is detected. The TCC will remain disabled whenever a misfire is detected. This allows the misfire diagnostic to evaluate the system.

Fuel Trim System Monitor Diagnostic Operation

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

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 the fuel trim counts in the weighted fuel trim cells exceed the specifications. A vehicle that has a fuel trim problem that is causing a concern under certain conditions but operates fine under other conditions may not set a fuel trim DTC. For example an engine that is idling high due to a small vacuum leak or an engine that is running rough due to a large vacuum leak may set an idle speed DTC or an HO2S DTC but not a fuel trim DTC.

A fuel trim DTC may be triggered by many different vehicle faults. Use all of the diagnostic information available when diagnosing a fuel trim fault.