The powertrain control module (PCM) is a precision unit consisting of a one chip microprocessor, an analog/digital (A/D) converter, and an input/output (I/O) unit. The PCM 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 performs the OBD 2 diagnostic tests of the emission related systems. The PCM supplies a buffered 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 PCM is also responsible for a self-diagnosis function and a fail-safe function.

The PCM (1) is located below the glove box, underneath the instrument panel, on the right side of the passenger compartment.

The PCM has an erasable programmable read-only memory (EPROM) computer chip that contains the calibration information used by the PCM to control fueling, idle speed, ignition timing, transaxle shifts, and vehicle emissions. The calibration information is based on various aspects of the vehicle, such as engine size, vehicle weight, transaxle type, final drive ratio, etc. The EPROM is programmed (flashed) with the calibration information that is critical to the proper operation of the PCM. The EPROM is soldered into the PCM and cannot be serviced separately. Replacement PCMs used for service require programming after installation.

The 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:

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. The PCM will store a diagnostic trouble code (DTC) when the PCM illuminates the MIL.

The PCM will turn OFF the MIL after 3 consecutive ignition cycles, in which the diagnostic runs, without the malfunction occurring. The DTC will remain stored in the PCM memory after the MIL is OFF.

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 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 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 occurs. The vehicle operator can teach the control module in order to regain some of the lost vehicle performance.

In order to teach the control module, ensure that the engine is at operating temperature and drive the vehicle at part throttle with moderate acceleration. The vehicle may also be operated at idle conditions until normal performance returns.

Important: Do not use the scan tool's Output Control feature until the vehicle is at normal operating temperatures. The PCM will not respond to a scan tool request for Output Controls until the engine coolant temperature is above 80°C (175°F). Failure to wait for normal operating temperatures can result in the incorrect diagnosis of a driveability problem.

The powertrain control module (PCM) can be directed by the Tech 2 scan tool to operate certain solenoids, valves, motors, and switches. This scan tool function is generally referred to as Output Controls. Output controls can be found under Special Functions on the Tech 2 scan tool. Operating a PCM controlled device with the scan tool should be limited to a maximum of ten seconds per test period.

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:

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.

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 forty (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 tool's clear the DTCs or clear the information function. Follow the instructions supplied by the scan tool manufacturer.

Notice: Turn the ignition OFF when installing or removing the PCM connectors and disconnecting or reconnecting the power to the PCM (battery cable, PCM pigtail, PCM fuse, jumper cables, etc.) in order to prevent internal PCM damage.

Important: Before replacing the powertrain control module (PCM) make sure that the PCM contains the correct calibrations for the model, equipment, and year of the vehicle being serviced. A PCM incorrectly programmed, or with the wrong calibrations, can set false or incorrect DTCs. Incorrect calibrations can also cause driveability symptoms and concerns.

The powertrain control module (PCM) is designed to withstand normal current draws associated with vehicle operations. Avoid overloading any circuit. Do not ground or apply voltage to any of the PCM circuits unless instructed to do so. Do not apply voltage to any of the PCM circuits when testing for an open or a short in an electrical circuit. The PCM circuits should only be tested using the J 39200 digital multimeter (DMM) or an equivalent. The PCM electrical connectors should remain connected to the PCM during testing.

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 vehicle's 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 vehicle's 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.

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

The PCM supplies a buffered 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 PCM is responsible for the control and operation of many output components. The PCM controls many components with an electronic switch 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:

(1)	Bank 1 Cylinder Head
(2)	HO2S Bank 1 Sensor 1
(3)	Bank 1 Warm-Up TWC Converter
(4)	HO2S Bank 1 Sensor 2
(5)	TWC Converter
(6)	HO2S Bank 2 Sensor 2
(7)	Bank 2 Warm-Up TWC Converter
(8)	HO2S Bank 2 Sensor 1
(9)	Bank 2 Cylinder Head

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 effect on vehicle emissions. Some of the primary system based diagnostics are listed below 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 used to monitor exhaust gas oxygen content. The HO2S 1 or pre-catalyst sensor is installed before the TWC converter. The HO2S 2 or post-catalyst sensor is installed after the TWC converter. 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 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 converter 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 HO2S. 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 in the TWC. A high oxygen storage capacity indicates a good catalyst. A low oxygen storage capacity indicates a failing catalyst. The TWC converter 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:

•	Any exhaust leaks

•	Any HO2S contamination

•	Any alternative fuels

Exhaust system leaks may cause any of the following results:

•	A false failure for a normally functioning or good catalyst.

•	Prevent a degraded or bad 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 and post-catalyst HO2S. 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 its 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 sensors voltage levels. If the activity of the post-catalyst oxygen sensor approaches that of the pre-catalyst oxygen sensor, the catalyst 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 oxygen storage capacity unless directed by the service manual.

A two 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 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 fuel sulfur content may be very high.

After-market 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 after-market 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)

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)

A degraded, or bad, 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 PCM 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 1,000 to 3,200 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.

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 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 both values are outside their 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 diagnostic information available when diagnosing a fuel trim fault.