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 check 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 display or on the SERVICE ENGINE SOON display. The MIL performs the following functions:
| • | Informs the driver that a problem has occurred and that the vehicle should be taken for service as soon as possible. |
| • | As a bulb check and system check, the MIL will illuminate with the ignition switch ON and the engine not running. 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 switch is ON. |
| • | If the MIL is not illuminated and the engine stalls, the MIL will not illuminate until the ignition switch is cycled OFF, then ON. |
Perform the Powertrain On Board Diagnostic (OBD) System Check first, when the following conditions are present:
| • | The MIL does not turn ON when the ignition switch is turned to the RUN position |
| • | The MIL remains ON while the engine is running |
| • | When you suspect a driveability problem |
Diagnostic Aids
An intermittent may be caused by the following conditions:
| • | Poor connections |
| • | Rubbed through wire insulation |
| • | Broken wire inside the insulation |
| • | Damaged harness |
| • | Improper mating |
| • | Improperly formed or damaged terminals |
| • | Poor terminal to wire connection |
Inspect all wiring and connections of the PCM harness.
Test Description
The numbers below refer to the step numbers on the diagnostic table:
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The MIL should be ON steady with the key 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 to 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.
| • | PCM error DTCs |
| • | System voltage DTCs |
| • | Component Level DTCs--For example, switches, sensor range and performance, sensor high voltage, sensor low voltage, and ODMs. |
| • | System level DTCs--For example Fuel trim, Misfire, EGR flow, TWC System, Idle Control System, HO2S Response, or HO2S Transition Time Ratio. |
Step | Action | Value(s) | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
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Important:
Is the MIL illuminated? | -- | |||||||||||
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 and Failure Records. Use the scan tool in order to display Diagnostic Trouble Codes (DTCs). Does the scan tool indicate any DTCs set? | -- | Go to Applicable DTC | ||||||||||
With a scan tool, compare the scan tool data to the Scan Tool Data List . Does the scan tool indicate PCM values equal to or within the typical values? | -- | System OK | Go to Component System Checks from Diagnostic Trouble Code (DTC) List/Type . |
