Circuit Description
The Powertrain On-Board Diagnostic (OBD) System Check must be the starting point for any driveability complaint diagnosis. Before using this procedure, you should perform a careful visual/physical inspection of the PCM and engine grounds for being clean and tight. The Powertrain On Board Diagnostic (OBD) System Check is an organized approach to identifying a problem created by an electronic engine control system malfunction.
MIL Operation
The malfunction indicator lamp (MIL) is located on the instrument panel (CHECK ENGINE) or (SERVICE ENGINE SOON). The MIL performs the following functions:
| • | It informs the driver that a problem has occurred and that the vehicle should be taken in for service as soon as possible. |
| • | As a bulb test and system test, the MIL will illuminate with the ignition ON, with the engine OFF. when the engine is started, the MIL will turn OFF. If the MIL remains ON, the self-diagnostic system has detected a problem. If the problem goes away, the MIL will turn OFF in most cases, but a diagnostic trouble code will remain stored. |
| • | If the MIL is illuminated, then the engine stalls, the MIL will remain Illuminated so long as the ignition is ON. |
| • | Now, if the MIL is not illuminated and the engine stalls, the MIL will not illuminate until the ignition is cycled OFF, then ON. |
Perform the Powertrain On Board Diagnostic (OBD) System Check first, when the following conditions are present:
| • | When the MIL does not illuminate when the ignition is turned to the RUN position |
| • | When the MIL remains illuminated while the engine is running |
| • | When you suspect a driveability problem |
Diagnostic Aids
An intermittent may be caused by a poor connection, rubbed through wire insulation or a broken wire inside the insulation. Inspect for poor connections or a damaged harness. Inspect the PCM harness and connectors for the following conditions:
| • | Improper mating |
| • | Broken locks |
| • | Improperly formed or damaged terminals |
| • | Poor terminal to wire connections |
| • | Damaged harness |
Test Description
The numbers below refer to the step numbers on the Diagnostic Table:
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The MIL should be Illuminated steady with the ignition ON, with the engine OFF. If not, the No Malfunction Indicator Lamp table should be used to isolate the malfunction.
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This test ensures that the PCM is capable of transmitting Class 2 serial data to the DLC and that the Class 2 data circuit is not open or shorted. If a problem is encountered and a malfunctioning scan tool is suspected, try the scan tool on another vehicle to verify operation. If a DLC problem exists, the DLC Diagnosis table should be used to diagnose the condition.
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If the engine will not start, the Engine Cranks But Will Not Run table should be used to diagnose the condition.
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The scan tool may display DTCs which are diagnosed in other diagnostic tables. Refer to DTC List for a complete list of DTCs supported by this vehicle application. If multiple DTCs are stored, diagnose each DTC according the following priority:
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A scan tool parameter which is not within the typical range may help to isolate the area which is causing the problem.
| • | System Voltage DTCs. |
| • | PCM Error DTCs. |
| • | U DTCs |
| • | Component Level DTCs (Switches, sensor range/performance, sensor high voltage, sensor low voltage, ODMs, etc.). |
| • | System level DTCs (Fuel trim, Misfire, TWC System, Idle Control System, HO2S Response or HO2S Transition Time Ratio). |
| • | BCM DTCs |
| • | VTD DTCs |
Step | Action | Value(s) | Yes | No | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
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Important:
Does the MIL illuminate? | -- | |||||||||
With the scan tool, attempt to display PCM data. Does the scan tool display PCM data? | -- | |||||||||
3 | Attempt to crank the engine. Does the engine crank? | -- | ||||||||
Attempt to start the engine. Does the engine start and continue to run? | -- | |||||||||
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Important: If the scan tool indicates any DTCs set, record Freeze Frame/Failure Records. With the scan tool display Diagnostic Trouble Codes (DTCs) including U, BCM, and VTD DTCs. Are any DTCs set? | -- | |||||||||
With the scan tool, compare the Scan Tool Data List data to the Scan Tool Data List . Does the scan tool indicate PCM values equal to or within the typical values? | -- | Go to Intermittent Conditions | Go to Symptoms |
