For 1990-2009 cars

## Powertrain Control Module Description General Description

The powertrain control module (PCM) (1) is a precision unit consisting of a one chip microprocessor, A/D (Analog/Digital) converter, and an I/O (Input/Output) unit. The PCM is an essential part of the electronic control system and is responsible for such major functions as control of the fuel injector, the idle air control (IAC) valve, the fuel pump relay, etc. The PCM is also responsible for a self-diagnosis function and a fail-safe function which are described in the following paragraphs.

The PCM is located below the instrument panel to the left of the steering column.

### Self-Diagnosis Function

The Powertrain Control Module (PCM) diagnoses troubles which may occur in the 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.

• | Heated Oxygen Sensor 1 (HO2S 1).

• | Heated Oxygen Sensor 2 (HO2S 2).

• | Engine Coolant Temperature (ECT) sensor.

• | Throttle Position (TP) sensor (including the CTP switch).

• | Vehicle Speed Sensor (VSS).

• | Intake Air Temperature (IAT) sensor.

• | Mass Air Flow (MAF) sensor.

• | Camshaft Position (CMP) sensor.

• | Exhaust Gas Recirculation (EGR) system.

• | Central Processing Unit (CPU) of PCM.

The PCM and the MIL operate as follows:

The MIL illuminates when the ignition switch is turned to the ON position (engine not running), regardless of the condition of the Sequential Multiport Fuel Injection (SFI) system. This is only to check the MIL circuit.

Once the engine starts and no faults are detected by the PCM, the MIL goes out. When the PCM detects a malfunction in one of the above areas, the PCM will illuminate the MIL in order to notify the driver of the occurrence of a fault.

### Fail-Safe Function

When a malfunction occurs within the Sequential Multiport Fuel Injection (SFI) system, the Powertrain Control Module (PCM) maintains control over the fuel injector, Idle Air Control (IAC) valve etc., on the basis of the calculated values and/or backup programs stored within the PCM.

This function is called the Fail-Safe Function. With this function, a certain level of engine performance is available even when a malfunction occurs, thereby avoiding complete loss of engine performance.

The systems covered are as follows:

• | The Engine Coolant Temperature (ECT) sensor

• | The Intake Air Temperature (IAT) sensor

• | The Throttle Position (TP) sensor

• | The Mass Air Flow (MAF) sensor

• | The Central Processing Unit (CPU) in PCM

• | The Fuel Level sensor.

• | The Vehicle Speed (VSS) sensor

• | The Closed Throttle Position (CTP) switch

• | The Barometric Pressure sensor

• | Transmission Range (TR) switch (4spd. A/T only)

• | Transmission shift solenoids

### ISO (9141-2) Serial Data

Regulations require that all automobile manufacturers establish a common communications system. The Tracker utilizes the International Organization for Standardization (ISO [9141-2]) communications system. It specifies the requirements for setting up the interchange of digital information between the on-board emission-related Control Modules of road vehicles and the OBD II scan tool as specified in J 1978. This communication is established to facilitate compliance with California Code of Regulation. 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.

### Data Link Connector (DLC) J1962

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 right 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.

### Powertrain Control Module Function

The powertrain control module (PCM) is designed to maintain exhaust emission levels while maintaining excellent driveability and fuel efficiency. The PCM supplies a buffered voltage to the various sensors and switches. The PCM controls most components with an electronic switch that completes a ground circuit when turned ON. The PCM controls the following operations:

• | Fuel control

• | Ignition Control (IC)

• | Automatic Transaxle shift functions

• | Evaporative Emission (EVAP) Purge

• | Exhaust Gas Recirculation (EGR)

• | A/C Compressor Clutch Cutout

### Use Of Circuit Testing Tools

Do not use a test light to diagnose the powertrain control electrical systems unless specifically instructed by the diagnostic procedures. Use J 35616 Connector Adapter Kit whenever the diagnostic procedures call for probing any connectors.

### PCM Service Precautions

The control module is designed to withstand normal current draws associated with vehicle operations. Avoid overloading any circuit. When testing for the opens or the shorts, do not ground or apply voltage to any of the control module circuits unless instructed to do so. These circuits should only be tested using a digital voltmeter J 39200. The control module connectors should remain connected to the control module while testing.

### Aftermarket (Add-On) Electrical And Vacuum Equipment

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

Do not connect any add-on vacuum equipment to this vehicle.

Connect add-on electrical equipment only to the vehicle's battery (power and ground). No connections are allowed to any other part of the vehicle's electrical system.

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

### Electrostatic Discharge (ESD) Damage

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 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, it is important to use care when handling and testing electronic components to avoid electrostatic charges that can cause electronic component damage.

### Maintenance Schedule

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

### Visual/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/physical inspection:

• | Inspect all the vacuum hoses for the following conditions:

• | Pinches

• | Cuts

• | Disconnects

• | Inspect all wires in the engine compartment for the following items:

• | Proper connections

• | Burned or chafed spots

• | Pinched wires

• | Contact with sharp edges

• | Contact with hot exhaust manifolds

• | This visual/physical inspection is very important. The visual/physical inspection must be done carefully and thoroughly.

### Basic Knowledge Of Tools Required

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

A basic understanding of hand tools is necessary to effectively use this section of the Service Manual.

### Oxygen Sensor Diagnosis

Diagnose the Fuel Control Heated Oxygen Sensor (HO2S 1) is diagnosed for the following conditions:

• | A Slow Response

• | An Inactive Signal (output steady at bias voltage--approx. 450 mV)

• | A Signal Fixed High

• | A Signal Fixed Low

Diagnose the Catalyst Monitor Heated Oxygen Sensor (HO2S 2) is diagnosed for the following functions:

• | The Heater Performance (time to activity on cold start)

• | A Signal fixed low during steady state conditions or power enrichment (hard acceleration when a rich mixture should be indicated).

• | A Signal fixed high during steady state conditions or decel fuel mode (deceleration when a lean mixture should be indicated).

• | An Inactive Sensor.

### Fuel Control Heated Oxygen Sensor (HO2S 1)

The main function of the fuel control oxygen sensor is to provide the control module with exhaust stream information to allow the proper fueling and the maintenance of emissions within the mandated levels. After the HO2S 1 reaches operating temperature, the sensor will generate a voltage, inversely proportional to the amount of oxygen present in the exhaust gases.

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

If the oxygen sensor pigtail wiring, the connector or the terminal are damaged, the entire oxygen sensor assembly must be replaced. Do not attempt to repair the wiring, the connector or the terminals. In order for the sensor to function properly, it must have clean reference air provided to it. This clean reference air is obtained through the oxygen sensor wire(s). Any attempt to repair the wires, the connectors, or the terminals could result in the obstruction of the reference air and thus degrade the oxygen sensor performance.

### HO2S Heater

The heated oxygen sensors are used to minimize the amount of time required to begin the closed loop fuel control operation and to allow accurate catalyst monitoring. The oxygen sensor heater greatly decreases the amount of time required for fuel control sensor HO2S to become active. The oxygen sensor heater is required by the catalyst monitor sensor HO2S 2 to maintain a sufficiently high temperature. The heater provides accurate exhaust oxygen content readings further from the engine.

### Catalyst Monitor Heated Oxygen Sensor (HO2S 2)

In order to control emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NOx), a three-way catalytic converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gases, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it to nitrogen. The PCM has the ability to monitor this process using the HO2S 1 and the HO2S 2 heated oxygen sensors.

The HO2S 1 sensor produces an output signal which indicates the amount of oxygen present in the exhaust gas entering the Three-Way Catalytic (TWC) converter. The HO2S 2 sensor produces an output signal which indicates the oxygen storage capacity of the catalyst; this in turn indicates the catalysts ability to convert exhaust gases efficiently. If the catalyst is operating efficiently, the HO2S 1 signal will be far more active than that produced by the HO2S 2 sensor.

In addition to the catalyst monitoring, the HO2S 2 heated oxygen sensor has a limited role in controlling fuel delivery. If the HO2S 2 signal indicates a high or low oxygen content for an extended period of time while in closed loop, the PCM will adjust the fuel delivery slightly to compensate.

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

### Catalyst Monitor Diagnostic Operation

The catalyst monitor diagnostic measures oxygen storage capacity. In order to do this, heated sensors are installed before and after the Three-Way Catalyst (TWC) converter. 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, its capacity to store and release oxygen generally degrades. The catalyst monitor diagnostic is based on an correlation between the conversion efficiency and the oxygen storage capacity. A good catalyst (e.g. 95 percent hydrocarbon conversion efficiency) will show a relatively flat output voltage on the post-catalyst Heated Oxygen 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/release capacity of the catalyst converter. A high oxygen storage capacity indicates a good catalyst. Low oxygen storage capacity indicates a failing catalyst. The TWC and HO2S 2 must be at operating temperature to achieve 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

• | Alternate fuels.

Exhaust system leaks may cause the following results:

• | May prevent a degraded catalyst from failing the diagnostic.

• | May cause a false failure for a normally functioning catalyst.

• | May prevent the diagnostic from running.

Some of the contaminants that may be encountered are phosphorus, lead, silica, and sulfur. The presence of these contaminants at any HO2S will prevent the TWC converter diagnostic from functioning properly.

### Three-Way Catalyst (TWC) Oxygen Storage Capacity

The PCM must monitor the Three-Way catalyst (TWC) for efficiency. In order to accomplish this, the control module monitors the pre-catalyst HO2S and post-catalyst HO2S 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 and releases oxygen during its normal reduction and oxidation process. The PCM will calculate the oxygen storage capacity using the difference between the pre catalyst and post catalyst oxygen sensors voltage levels.

When ever the voltage levels of the post-catalyst (2) oxygen sensor nears the voltage levels that 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 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.

### 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 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 drive train. This torque can intermittently decrease the crankshaft rotational velocity. This may be falsely detected as a misfire.

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 drive wheel inputs on crankshaft rotation.

When the TCC has been disabled as a result of misfire detection, it 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 reevaluate the system.

### Fuel Trim System Monitor Diagnostic 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 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 either value is outside the thresholds, a rich or lean fuel Trim DTC will set.

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

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

### 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 (i.e. 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:

• | The Vehicle Speed Sensor (VSS)

• | The Crankshaft Position (CKP) sensor

• | The Throttle Position (TP) sensor

• | The Closed Throttle Position (CTP) switch

• | The Engine Coolant Temperature (ECT) sensor

• | The Camshaft Position (CMP) sensor

• | The Manifold Absolute Pressure (MAP) sensor

• | The Heated Oxygen Sensors (HO2S)

• | The Mass Air Flow (MAF) sensor

• | The Fuel Tank Pressure Sensor

• | The Fuel Level Sensor

In addition to the circuit continuity and performance 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 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 EVAP Canister Purge Valve

• | The Electronic Transaxle controls

• | The VSS output (A/T only)

• | The MIL control

• | The A/C compressor inhibit function

• | The cruise control inhibit function

### Wiring Harness Service

Replace the wire harness 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.

Molder-on connectors require complete replacement of the connector. Splice a new connector into the harness. Replacement connectors and terminals are listed in the Group 8.965 in the Standard Parts Catalog.

For wiring repair, refer to Wiring Repair.

### Connector Components

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 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 adapt the fuse holder to a J 39200 DMM for diagnosis.

Open circuit 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.

## Powertrain Control Module Description Common OBD ll Terms

Table 1: | Storing Freeze Frame and Failure Record Data

The 1998 Chevrolet Tracker conforms to all OBD II mandated emission regulations. The powertrain control module (PCM) is responsible for monitoring and modifying engine controls in order to meet all OBD II requirements. Certain minimum criteria for how the PCM monitors and diagnoses the emisssion system is specified in the OBD II regulations. The following information describes the common operations and terms of the OBD II regulations that the Tracker PCM is designed to adhere to.

### Diagnostic

The word diagnostic, when used as a noun, refers to any on-board test run by the vehicle's powertrain control module (PCM). A diagnostic is simply a test run on a system or component in order to determine if the system or the component is operating according to specifications. The following list defines the major vehicle on-board diagnostics. Depending on the emission requirements in the area of vehicle sale, certain diagnostics listed below may not apply.

• | Misfire

• | Oxygen sensors

• | Oxygen sensor heaters

• | EGR

• | Catalyst monitoring

### Enable Criteria

The term enable criteria is engineering language for the conditions necessary for a given diagnostic test to run. Each diagnostic has a specific list of conditions which must be met before the diagnostic will run. Enable criteria is another way of saying conditions required.

The enable criteria for each diagnostic is listed on the first page of the DTC description under the heading Conditions for Setting the DTC. The enable criteria varies with each diagnostic, and typically includes, but is not limited to the following items:

• | Engine Speed (RPM)

• | Vehicle Speed (A/T only)

• | Engine Coolant Temperature (ECT)

• | Mass Air Flow (MAF)

• | Barometric Pressure (BARO)

• | Intake Air Temperature (IAT)

• | Throttle Position (TP)

• | High Canister Purge

• | Fuel-trim

• | Torque Converter Clutch (TCC) status

### Trip

Technically, a trip is a key on-run-key off cycle in which all the enable criteria for a given diagnostic are met, allowing the diagnostic to run. Unfortunately, this concept is not quite that simple. A trip is official when all the enable criteria for a given diagnostic are met. Because the enable criteria vary from one diagnostic to another, the definition of a trip varies as well. Some diagnostics are run when the vehicle is at operating temperature. Some diagnostics are run when the vehicle first starts up. Some diagnostics require that the vehicle is cruising at a steady highway speed. Some diagnostics run only when the vehicle is at idle. Some diagnostics function with the torque converter clutch disabled. Some diagnostics run only immediately following a cold engine start-up.

A trip then, is defined as a key on-run-key off cycle in which the vehicle was operated in such a way as to satisfy the enable criteria for a given diagnostic. This diagnostic will consider this cycle to be one trip. However, another diagnostic with a different set of enable criteria (which were not met) during this driving event, would not consider it a trip. No trip will occur for that particular diagnostic until the vehicle is driven in such a way as to meet all the enable criteria.

### Passive and Active Diagnostic Tests

A passive test is a diagnostic test which simply monitors a vehicle system or a vehicle component. An active test actually takes some sort of action when the performing diagnostic functions. An active test is often in response to a failed passive test. For example, the exhaust gas recirculation (EGR) diagnostic active test may force the EGR valve open during a closed throttle deceleration maneuver. Or the EGR diagnostic active test may force the EGR valve closed during a period of steady speed driving. Either action should result in a change in the manifold pressure.

### Warm-Up Cycle

A warm-up cycle means that the engine temperature must reach a minimum of 70°C (160°F) and rise at least 22°C (40°F) over the course of a trip.

### Freeze Frame and Failure Records

Freeze Frame is an element of the Diagnostic Management System that stores various vehicle information in the PCM memory at the moment an emissions-related fault occurs that commands the MIL On. Freeze Frame data is useful in identifying the cause of an emission-related fault. Freeze Frame data is accessed by the scan tool and can be saved with the Capture Data feature. Refer to Storing And Erasing the Freeze Frame Data in Powertrain Control Module Diagnosis for more detailed information.

The PCM can store one Freeze Frame and save the recorded data of three additional Freeze Frames as Failure Records. Therefore the PCM can store up to 4 frames of Freeze Frame and Failure Record data. The 1st frame stores data of the fault that was detected first. The data from the 1st detected fault is also stored in the 2nd frame as a Failure Record. The Failure Record data stored in the 2nd frame is permanent, and will not change when a new fault is detected. The 1st frame of Freeze Frame data will remain unchanged unless a fault of a higher priority occurs. A misfire fault (DTC P0300-P0304) or a fuel trim fault (DTC P0171 and P0172) will replace the data in the 1st frame of Freeze Frame data because these DTCs have a higher priority under OBD 2 rules. The 2nd through 4th frames of Failure Records will store fault data in the order that the faults were detected regardless of the fault's priority. Utilizing the 4 frames of Freeze Frame and Failure Record data can provide information on the order in which the faults were first detected. The following table indicates how Freeze Frame and Failure Record data is stored when two or more faults are detected.

#### Storing Freeze Frame and Failure Record Data

Order | Fault | Frame 1 | Frame 2 | Frame 3 | Frame 4
--- | --- | --- | --- | --- | ---
-- | No Fault detected | No Freeze Frame data | No Failure Record data | No Failure Record data | No Failure Record data
1 | DTC P0400 | P0400 data stored | P0400 data stored | -- | --
2 | DTC P0171 | P0171 data replaces P0400 data | P0400 data still stored | P0171 data stored | --
3 | DTC P0300 | P0171 data still stored | P0400 data still stored | P0171 data still stored | P0300 data stored
4 | DTC P0301 | P0171 data still stored | P0400 data still stored | P0171 data still stored | P0300 data still stored

### Diagnostic Information

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

There is a continuous self-diagnosis on certain control functions. This diagnostic capability is complimented by the diagnostic procedures contained in this service information. The language of communicating the source of the malfunction is a system of diagnostic trouble codes (DTCs). When a malfunction is detected by the powertrain control module, a DTC is set and the malfunction indicator lamp (MIL) lamp is illuminated.

### Malfunction Indicator Lamp (MIL)

The malfunction indicator lamp (MIL) looks the same as the MIL you may be already familiar with (Service Engine Soon or Check Engine). OBD II regulations require that the MIL illuminate according to a strict set of guidelines. Under OBD II the MIL is turned on when the PCM detects a malfunction that will impact the vehicle emissions.

The MIL is controlled by the PCM. The MIL will be illuminated if an emissions-related diagnostic test indicates a malfunction has occurred. The MIL will stay illuminated until the system or the component passes the same test for three consecutive trips.

A vehicle that is experiencing a misfire malfunction that may cause damage to the three-way catalytic converter (TWC) will flash the MIL once per second. The MIL will continue to flash once per second until the vehicle is outside of the speed and the load conditions that may cause damage to the TWC catalyst. The MIL will stop flashing and remain on steady once the vehicle is outside of the speed and the load conditions that may cause damage to the TWC catalyst.

### Extinguishing the MIL

The powertrain control module (PCM) will turn Off the malfunction indicator lamp (MIL) after three consecutive trips that a test passed has been reported for the diagnostic test that originally caused the MIL to illuminate.

The diagnostic trouble code (DTC) will remain in the PCM memory and the Freeze Frame record until forty (40) warm-up cycles have been completed and no faults exist.

An MIL that was illuminated by either a fuel-trim DTC or a misfire-related DTC has additional requirements that must be met in order to turn Off the MIL. The additional requirements are as follows:

• | The diagnostic tests that are passed must occur within 375 RPM of the RPM data stored at the time the last test failed.

• | The diagnostic tests that are passed must occur within 0 to 20 percent of the engine load that was stored at the time the last test failed.

• | The diagnostic tests that are passed must occur at engine temperature conditions (warmed up or warming up) that are similar to those stored at the time the last test failed.

Meeting these requirements ensures that the fault which turned on the MIL has been corrected.

### DTC Types

DTCs are categorized by type. The type indicates the action the PCM will take in recording the DTC failure and illuminating the MIL. The following table indicates what action the PCM takes for the 3 different DTC types when a DTC failure occurs.

DTC Type | MIL Illumination | Freeze Frame Stored | Failure Records Stored*
--- | --- | --- | ---
A | Yes | Yes | Yes
B | Yes (with two fails) | Yes (on second failure) | Yes (on second failure)
C | No | No | No
* The PCM in this vehicle stores up to three Failure Records

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 DTC List table in Specifications for the type of DTC the particular PCM supports.

Type C DTCs that DO NOT display a message were formerly referred to as Type D.

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

The powertrain control module (PCM) has the capability of alerting the vehicle operator to potentially damaging levels of misfire. A misfire condition that could potentially damage the catalytic converter as a result of high misfire levels will command the PCM to flash the malfunction indicator lamp (MIL) at a rate of once per second. This once per second flashing of the MIL will occur during the entire time that the catalyst damaging misfire condition is present. This flashing of the MIL is unique to the misfire diagnostic.

### Maintenance Schedule

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