Powertrain Control Module Description Common OBD II Systems

The Chevrolet Metro 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 Metro PCM is designed to adhere to.

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.

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:

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.

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.

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 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-P0303) 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.

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.

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.

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:

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

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.

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.

There are two type C DTCs. The C1 type DTC will illuminate an information lamp or display a message. The type C0 DTC DOES NOT illuminate an information lamp or display a message. Type C0 DTCs 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.

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.

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.