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

Powertrain Control Module

The Powertrain Control Module (PCM) is located in the engine compartment. The PCM is the control center of the vehicle.

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

The PCM supplies either 5.0 or 12.0 volts to power various sensors or switches. This is done through resistances in the PCM. The resistance is so high in value that a test lamp will not illuminate when connected to the circuit. In some cases, even an ordinary shop voltmeter will not give an accurate reading because its resistance is too low. Therefore, a digital voltmeter (J 39200) with a minimum of 10 megaohms input impedance is required to ensure accurate voltage readings.

The PCM controls output circuits such as the injectors, IAC, cooling fan relays, etc. by controlling the ground or the power feed circuit through transistors or a device called an Output Driver Module.

Torque Management

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

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

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

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

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

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

PCM Function

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

PCM Service Precautions

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

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.

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

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

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

Engine Controls Information

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

The engine controls Information contains the following:

The Component System includes the following items:

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

Maintenance Schedule

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

Visual and Physical Underhood Inspection

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

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

System Status and Drive Cycle For Inspection/Maintenance

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

Several states require that the I/M (OBD ll system) pass on-board tests for the major diagnostics prior to having a vehicle emission inspection. This is also a requirement to renew license plates in some areas.

Using a scan tool, the technician can observe the System Status (complete or not complete) in order to verify that the vehicle meets the criteria to comply with local area requirements. Using the System Status display, any of the following systems or combination of systems may be monitored for I/M Readiness:

Important: The System Status display indicates only whether or not the test has been completed. The System Status display does not necessarily mean that the test has passed. If a Failed Last Test indication is present for a DTC associated with one of the above systems, diagnosis and repair is necessary in order to meet the I/M requirement. Verify that the vehicle passes all of the diagnostic tests associated with the displayed System Status prior to returning the vehicle to the customer. Refer to the Typical Drive Cycle table (more than one drive cycle may be needed) to use as a guide to complete the I/M System Status tests.

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

Typical Drive Cycle

Diagnostic Time Schedule for I/M Readiness
Vehicle Drive Status	What is Monitored?
Cold Start, coolant temperature less than 50°C (122°F)	--
Idle 2.5 minutes in 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 braking. 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 for the following conditions:

Diagnose the Catalyst Monitor Heated Oxygen Sensors for the following functions:

Heated Oxygen Sensors (Pre-Catalyst)

The main function of the Pre-Catalyst Heated Oxygen Sensor (HO2S) is to provide the PCM with exhaust stream information in order to maintain proper fueling to hold emissions within acceptable levels. These oxygen sensors are always located between the exhaust manifold and the catalytic converter. After the sensor reaches the operating temperature, the sensor generates a voltage inversely proportional to the amount of oxygen present in the exhaust gases.

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

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

Catalyst Monitor Heated Oxygen Sensors

In order to control emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NOx), the system uses a three-way catalytic converter. The catalyst promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it to nitrogen. Catalyst Monitor HO2S ( Post Catalyst HO2S) are always located downstream of the catalytic converter.

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

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

Catalyst Monitor Diagnostic Operation

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

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

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

The post-catalyst HO2S is used to measure the oxygen storage and release capacity of the catalyst. A high oxygen storage capacity indicates a good catalyst. Low oxygen storage capacity indicates a failing catalyst. The TWC and the HO2S must be at operating temperature in order to achieve reliable oxygen sensor voltages like those shown in the Three-Way Catalyst Oxygen Storage Capacity graphic.

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

The catalyst monitor diagnostic is sensitive to the following conditions:

Exhaust system leaks may cause the following:

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

Three-Way Catalyst Oxygen Storage Capacity

The PCM must monitor the Three-Way Catalyst system (TWC) for efficiency. In order to accomplish this, the PCM monitors the pre-catalyst and post-catalyst oxygen sensors. When the TWC is operating properly, the post-catalyst (2) oxygen sensor will have significantly less activity than the pre-catalyst (1) oxygen sensor. The TWC stores oxygen during its normal reduction and oxidation process. The TWC releases oxygen during its normal reduction and oxidation process. The PCM calculates the oxygen storage capacity using the difference between the pre-catalyst and post-catalyst oxygen sensor's voltage levels.

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

Aftermarket HO2S characteristics may be different from the original equipment manufacturer sensor. This may lead to a false pass or a false fail of the catalyst monitor diagnostic. Similarly, if an aftermarket catalyst does not contain the same amount of precious metal content as the original part, the correlation between oxygen storage and conversion efficiency may be altered enough to set a false DTC.

Misfire Monitor Diagnostic Operation

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

For a non-catalyst damaging misfire, the diagnostic is required to monitor a misfire present for between 1000-3200 engine revolutions.

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

Rough roads may cause false misfire detection. A rough road applies sudden torque variations to the drive wheels and drivetrain. This torque can intermittently decrease the crankshaft rotational velocity. The ABS system detects uneven speed between the vehicles wheels and sends data via the serial data bus to the PCM to disable the misfire monitor until the rough road is no longer detected.

On 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 effect of the drive wheel inputs on crankshaft rotation.

When the TCC is disabled as a result of misfire detection, the TCC will re-enabled after approximately 3200 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.

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

A current and a history misfire counter is maintained for each cylinder. The misfire current counters (Misfire Cur #1 - 8) indicate the number of firing events out of the last 200 cylinder firing events which were misfires. The misfire current counters displays real time data without a misfire DTC stored. The misfire history counters (Misfire Hist #1 - 8) indicate the total number of cylinder firing events which were misfires. The misfire history counters displays 0 until the misfire diagnostic has failed and a DTC P0300 is set. Once the misfire DTC sets, the misfire history counters will be updated every 200 cylinder firing events. The Misfire counters graphic illustrates how these misfire counters are maintained.

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

If two cylinders in sequential firing order are both misfiring, the first misfiring cylinder will accumulate misfires in its buffer but the second misfiring cylinder will not. This is because the PCM compares a misfiring cylinder with the cylinder 90 degrees prior to it in the firing order. Therefore the PCM would be comparing crankshaft speed of the second misfiring cylinder to an already suspect cylinder. The PCM however, will be able to detect both misfiring cylinders after the engine exceeds 2000 RPM. This is because the PCM then starts to compare misfires to the opposing cylinder rather than the previous cylinder in the firing order.

Use Techline equipment to monitor the misfire counter data on applicable vehicles. Knowing which specific cylinder(s) misfire can lead to the root cause. Using the information in the misfire counters, will identify which cylinder(s) are misfiring. If the counters indicate cylinders number 1 and 4 misfired, look for a circuit or component common to both cylinders.

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

Fuel Trim System Operation

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

Comprehensive Component Monitor Diagnostic

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

Input Components

The PCM monitors the input components for circuit continuity and out-of-range values. This includes performance checking. Performance checking refers to indicating a fault when the signal from a sensor does not seem reasonable (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:

In addition to the circuit continuity and rationality check, the ECT sensor is monitored for its ability to achieve a steady state temperature to enable closed loop fuel control.

Output Components

Diagnose the output components for the proper response to PCM commands. Components where functional monitoring is not feasible will be monitored for circuit continuity and out-of-range values if applicable.

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