For 1990-2009 cars

### 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. The MIL performs the following functions:

• | 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 and 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 and the engine stalls, the MIL will remain Illuminated so long as the ignition is on.

• | If the MIL is not illuminated and the engine stalls, the MIL will not illuminate until the ignition is cycled off and on.

Perform the Powertrain On-Board Diagnostic (OBD) System Check first, when the following conditions are present:

• | The MIL does not illuminate when the ignition is turned to the RUN position

• | 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 improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connections, and a damaged harness.

### Test Description

The numbers below refer to the step numbers on the diagnostic table:

1. The MIL should be Illuminated steady with the ignition on and the engine off. If not, the No Malfunction Indicator Lamp table should be used in order to isolate the malfunction.

2. 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.

3. If the engine will not start, the Engine Cranks But Will Not Run table should be used to diagnose the condition.

4. The scan tool may display DTCs which are diagnosed in other diagnostic tables. Refer to the DTC List for a complete list of the DTCs supported by this vehicle application. If multiple DTCs are stored, diagnose each DTC according the following priority:

• | The system voltage DTCs

• | The PCM error DTCs

• | The "U" DTCs

• | The component level DTCs

• | The system level DTCs

• | The Body Control Module (BCM) DTCs

• | The vehicle theft deterrent (VTD) DTCs

5. A scan tool parameter which is not within the typical range may help to isolate the area that is causing the problem.

#### Powertrain on--Board Diagnostic System Check

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Important:

• | Research the applicable service bulletins before proceeding with this diagnostic procedure.

• | Do not perform this test if no driveability condition exists.

• | Unless instructed, do not clear any DTCs or turn off the ignition.

1. Connect a scan tool.
2. Turn on the ignition, leaving the engine off.
3. Observe the Malfunction Indicator Lamp (MIL).

Does the MIL illuminate?

--

Go to Step 2

Go to Malfunction Indicator Lamp (MIL) Inoperative

2 | Use the scan tool in order to attempt to display the PCM data. Does the scan tool display the PCM data? | -- | Go to Step 3 | Go to Data Link Connector Diagnosis
3 | Attempt to crank the engine. Does the engine crank? | -- | Go to Step 4 | Go to Diagnostic System Check - Theft Deterrent in Theft Deterrent
4 | Attempt to start the engine. Does the engine start and continue to run? | -- | Go to Step 5 | Go to Engine Cranks but Does Not Run
5 | Important: If the scan tool indicates any DTCs set, record the Freeze Frame and Failure Records. With the scan tool display the DTCs including any U, BCM, and VTD DTCs. Are any DTCs set? | -- | Go to Diagnostic Trouble Code (DTC) List/Type | Go to Step 6
6 | Use the scan tool in order to compare the Scan Tool Data List data to the Scan Tool Data List . Does the scan tool indicate the PCM values are equal to or within the typical values? | -- | System OK | Go to Component System Checks