The powertrain control module (PCM), located under the instrument panel, is the control center of the fuel injection system. The PCM constantly looks at the information from various sensors, and controls the systems that affect emission or engine performance. The PCM also performs the diagnostic function of the system. The PCM can recognize operational problems, alert the driver through the malfunction indicator lamp (MIL), and store a diagnostic trouble code (DTC) or DTCs which identify the problem areas to aid the technician in making repairs.

This assembly contains the functions of the electrically erasable programmable read-only memory (EEPROM) and is a permanent part of the PCM. The EEPROM contains the calibrations needed for a specific vehicle applications and is serviced only through a re-programming procedure.

The PCM supplies either 5 volts or 12 volts to power various sensors or switches. This is done through resistances in the PCM which are 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 10 megohm input impedance digital multimeter (DMM) is required to assure accurate voltage readings.

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

Do not use a circuit 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.

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 a DMM while the control module electrical connectors remain connected to the control module.

The aftermarket (add-on) electrical and vacuum equipment is defined as any equipment installed on a vehicle after leaving the factory that connects to the electrical system or the vacuum system 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 eliminate all of the aftermarket electrical equipment from the vehicle. After this is done, if the problem still exists, diagnose the problem in the normal manner.

Notice: Do not touch the connector pins or soldered components on the circuit board in order to prevent possible electrostatic discharge (ESD) damage to the PCM.

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.

Refer to the General Motors Maintenance Schedule in Maintenance and Lubrication for the maintenance that the owner or technician should perform in order to retain emission control performance.

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. Preform the inspection carefully and thoroughly.

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.

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 in order to verify that the vehicle meets the criteria which comply 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 diagnostic trouble code (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 (more than one drive cycle may be needed) to use as a guide to complete the I/M 240 System Status tests.

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.

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.

Diagnose the fuel control oxygen sensors (O2S) for the following conditions:

Diagnose the heated oxygen sensors (HO2S 2) for the following functions:

The main function of the fuel control heated oxygen sensor (HO2S) 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. This clean air reference is obtained by the oxygen sensor wires. Any attempt to repair the wires, the connectors, or the terminals could result in the obstruction of the air reference. Any attempt to repair the wires, the connectors, or the terminals could degrade oxygen sensor performance.

The oxygen sensor heaters are required by catalyst monitor sensors to maintain a sufficiently high temperature which allows accurate exhaust oxygen content readings further from the engine.

In order to control emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), the system uses a 3-way catalytic converter. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting the HC and the CO into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting the NOx into nitrogen.

The control module has the ability to monitor this process using the heated oxygen sensor (HO2S). The HO2S produces an output signal which indicates the oxygen storage capacity of the catalyst. This in turn indicates the catalysts ability to convert the exhaust gases efficiently. If the catalyst is operating efficiently, the O2S signal will be far more active than that produced by the HO2S.

The OBD II catalyst monitor diagnostic measures oxygen storage capacity. In order to do this, the heated sensors are installed before and after the 3-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, 95 percent hydrocarbon conversion efficiency, shows a relatively flat output voltage on the post-catalyst HO2S. A degraded catalyst, 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 and HO2S 2 must be at operating temperature in order to achieve correct oxygen sensor voltages like those shown in the post-catalyst HO2S 2 Outputs graphic.

The catalyst monitor diagnostic is sensitive to the following conditions:

Exhaust system leaks may cause the following results:

Some of the following contaminants that may be encountered :

The presence of these contaminants prevents the TWC diagnostic from functioning properly.

The control module must monitor the three-way catalyst system (TWC) 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 catalysts normal reduction and oxidation process. The TWC releases oxygen as needed during the catalysts 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 directed by the electronic 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 at the inputs from the pre and post oxygen sensor (O2S) more closely 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.

(1)	EVAP Vent Solenoid
(2)	EVAP Vent Solenoid Ignition Feed Circuit Terminal
(3)	EVAP Vent Solenoid Control Circuit Terminal
(4)	EVAP Vent Solenoid Filter
(5)	EVAP Vapor Lines
(6)	Fuel Tank Pressure Sensor
(7)	Fuel Tank Pressure Sensor Ground Circuit Terminal
(8)	Fuel Tank Pressure Sensor Signal Circuit Terminal
(9)	Fuel Tank Pressure Sensor Circuit 5 Volt Reference Circuit Terminal
(10)	Fuel Filler Pipe
(11)	Check Valve (Spitback)
(12)	Modular Fuel Sender Assembly
(13)	Fuel Limiter Vent Valve (FLVV)
(14)	Pressure/Vacuum Relief Valve (Optional)
(15)	EVAP Canister
(16)	EVAP Purge Solenoid Ignition Feed Circuit Terminal
(17)	EVAP Purge Solenoid Control Circuit Terminal
(18)	Intake Manifold Vacuum Port
(19)	EVAP Purge Solenoid
(20)	EVAP Service Port

The evaporative emission (EVAP) system uses a fuel tank pressure (FTP) sensor, located on top of the fuel tank sender unit, in order to detect when the EVAP system is purging. The fuel tank pressure sensor detects the flow from the engine through the EVAP canister purge valve. When the EVAP system is not purging, the fuel tank pressure sensor will detect no change in the fuel tank vacuum. When the EVAP system is purging, the sensor will detect a vacuum in the fuel tank. A change in pressure will be displayed on the scan tool. If a leak, an obstruction, or a malfunctioning sensor are present, one of the following DTCs will set:

•

DTC P0440

•

DTC P0442

•

DTC P0446

•

DTC P0452

•

DTC P0453

•

DTC P1441

Misfire Monitor Diagnostic Operation

The misfire monitor diagnostic is based on the crankshaft rotational velocity, reference period, variations. The control module determines the crankshaft rotational velocity using the crankshaft position (CKP) sensor and the camshaft position (CMP) 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 is required to monitor a misfire present for between 1,000-3,200 engine revolutions.

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

Rough roads may cause a false misfire detection. A rough road applies torque to the drive wheels and the drive train. This torque can intermittently decrease the crankshaft rotational velocity. The control module detects this as a false misfire.

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

When the TCC has disabled as a result of misfire detection, the TCC will re-enabled after approximately 3,200 engine revolutions if no misfire is detected. The TCC remains disabled whenever the misfire is detected, with or without a DTC set. This allows the misfire diagnostic to reevaluate the system.

During a transmission high temperature condition, the misfire diagnostic will disable and the TCC will operate normally. This avoids further increasing the temperature of the transmission.

Misfire Counters

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

A current and a history misfire counter is maintained for each cylinder. The misfire current counters (Misfire Cur #1-4) 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 diagnostic trouble code (DTC) stored. The misfire history counters (Misfire Hist #1-4) indicate the total number of cylinder firing events which were misfires. The misfire history counters displays 0 until the misfire diagnostic has failed and a DTC P0300 is set. Once the misfire DTC sets, the misfire history counters will be updated every 200 cylinder firing events.

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

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

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

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

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

•	A contaminated fuel

•	The engine running out of fuel

•	Any fuel-fouled spark plugs

•	A basic engine fault

Fuel Trim System Monitor Diagnostic Operation

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

In order to meet OBD ll requirements, the control module uses weighted fuel trim cells in order to determine the need to set a fuel trim DTC. A fuel trim DTC can only be set if fuel trim counts in the weighted fuel trim cells exceed specifications. This means that the vehicle could have a fuel trim problem which is causing a concern under certain conditions, engine idle high due to a small vacuum leak or rough due to a large vacuum leak, while the vehicle operates fine at other times. No fuel trim DTC would set, although an engine idle speed DTC or O2S DTC may set. Remember, use a scan tool in order to observe fuel trim counts while the problem is occurring.

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

Comprehensive Component Monitor Diagnostic

The comprehensive component monitoring diagnostics are required to monitor emissions-related input and output Powertrain components. The CARB OBD II Comprehensive Component Monitoring List of Components Intended To Illuminate The MIL is a list of components, features, or functions that could fall under this requirement.

Input Components

The control module monitors the input components for circuit continuity and out-of-range values. This includes performance checking. Performance checking refers to indicating a fault when the signal from a sensor does not seem reasonable, for example, a throttle position (TP) sensor that indicates high throttle position at low engine loads or manifold absolute pressure (MAP) sensor 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 knock sensor (KS)

•	The throttle position (TP) sensor

•	The engine coolant temperature (ECT) sensor

•	The camshaft position (CMP) sensor

•	The manifold absolute pressure (MAP) sensor

•	The mass air flow (MAF) sensor

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

Output Components

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

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

•	The idle air control (IAC) motor

•	The evaporative emission (EVAP) canister purge valve

•	The electronic transmission controls

•	The A/C relay

•	The cooling fan relay

•	The vehicle speed sensor (VSS) output

•	The malfunction indicator (MIL) control

•	The cruise control inhibit, if applicable

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

Important: Not all vehicles have these components.

•	The transmission range (TR) mode pressure switch

•	The transmission turbine speed sensor (HI/LO)

•	The transmission vehicle speed sensor (HI/LO)

•	The ignition sensor (Cam Sync, Diag)

•	The ignition sensor Hi Res (7x)

•	The knock sensor (KS)

•	The engine coolant temperature (ECT) sensor

•	The intake air temperature (IAT) sensor

•	The throttle position (TP) sensor A, B

•	The manifold absolute pressure (MAP) sensor

•	The mass air flow (MAF) sensor

•	The automatic transmission temperature sensor

•	The transmission torque converter clutch (TCC) control solenoid

•	The transmission TCC enable solenoid

•	The transmission shift solenoid A

•	The transmission shift solenoid B

•	The transmission 3/2 shift solenoid

•	The ignition control (IC) system

•	The idle air control (IAC) coil

•	The evaporative emission (EVAP) purge vacuum switch

•	The EVAP canister purge

Wiring Harness Service

The control module harness electrically connects the control module to the various solenoids, switches, and sensors in the vehicle engine compartment and the passenger compartment.

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. Replacement connectors and terminals are listed in Group 8.965 in the Standard Parts Catalog.

For wiring repair, 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 testing.

Never probe through the Weather-Pack seals.

The J 35616-A Connector Test Adapter Kit , or the equivalent, contains an assortment of flexible connectors used in order to probe the terminals during the 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.