For 1990-2009 cars

Refer to Engine Controls Schematics .

### 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 and is displayed as CHECK ENGINE lamp.

#### MIL Function

• | The MIL informs the driver that a malfunction has occurred and the vehicle should be taken in for service as soon as possible

• | The MIL illuminates during a bulb test and a system test

• | A DTC will be stored if a MIL is requested by the diagnostic

#### MIL Illumination

• | The MIL will illuminate with ignition switch ON and the engine not running

• | The MIL will turn OFF when the engine is started

• | The MIL will remain ON if the self-diagnostic system has detected a malfunction

• | The MIL may turn OFF if the malfunction is not present

• | If the MIL is illuminated and 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.

#### Powertrain On Board Diagnostic (OBD) System Check

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

• | When the MIL does not turn ON when the ignition switch is turned to the RUN position. See above for MIL Operation

• | When the MIL remains ON while the engine is running

• | When the MIL is flashing while the engine is running

• | When a driveability symptom is determined

### Diagnostic Aids

An intermittent may be caused by a faulty connection, rubbed through wire insulation, or a wire broken inside the insulation. Inspect for faulty connections or a damaged harness. Inspect the PCM harness and connectors for improper mating, broken locks, improperly formed or damaged terminals, faulty terminal to wire connection, and damaged harness. Refer to Intermittent Conditions .

### Test Description

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

1. The MIL should be ON steady with the key ON / engine OFF. If not, the No Malfunction Indicator Lamp table should be used to isolate the malfunction.

2. This test ensures that the PCM is capable of transmitting Class II serial data to the DLC and that the Class II 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 4T65-E Automatic Transaxle Diagnosis. 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:

• | PCM Error DTCs

• | System Voltage DTCs

• | Component Level DTCs (switches, sensor range/performance, sensor high voltage, sensor low voltage, ODMs, etc.)

• | System level DTCs (Fuel trim, Misfire, EGR flow, TWC System, Idle Control System, HO2S Response or HO2S Transition Time Ratio)

• | System Reference DTCs

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

Step | Action | Values | Yes | No
--- | --- | --- | --- | ---
1 | Important: :

• | Identify any applicable service bulletins before proceeding with the diagnosis.

• | Do not turn OFF the ignition when performing this diagnostic table.

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

• | Unless instructed, do not clear any DTCs.

1. Turn ON the ignition, with the engine OFF.
2. Observe the malfunction indicator lamp (MIL).

Is the MIL illuminated?

--

Go to Step 2

Go to Malfunction Indicator Lamp (MIL) Inoperative

2 |

1. Turn OFF the ignition.
2. Install a scan tool.
3. Turn ON the ignition, with the engine OFF.
4. With a scan tool, attempt to display PCM data.

Does the scan tool display PCM data?

--

Go to Step 3

Go to Data Link Connector Diagnosis

3 | Attempt to start the engine Did the engine crank? | -- | Go to Step 4 | Go to Starter Inhibit Relay Circuit Diagnosis
4 | Did 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 Freeze Frame/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 | Go to Step 6
6 | With a scan tool, compare the PCM data to the Scan Tool Data List . Does the scan tool indicate PCM values equal or within to the typical values? | -- | System OK | Go to Component System Checks