Diagnostic System Check

Diagnostic Chart:

Engine Controls Schematics: Power, Ground, MIL and Data Link:

SYSTEM 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
Inspect for the following:

IMPORTANT: Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly.

The gaskets prevent contaminate intrusion.
^ Poor terminal connection.
Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Use a corresponding mating terminal to test for proper tension. Refer to Diagrams for diagnosis and repair.
^ Damaged harness.
Inspect the wiring harness for damage. If the harness inspection does not reveal a problem, observe the display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the scan tool display may indicate the location of the fault. Refer to Diagrams for diagnosis and repair.
^ Inspect the PCM and the engine grounds for clean and secure connections.

If the DTC is determined to be intermittent, reviewing the Failure Records can be useful in determining when the DTC was last set.

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 Automatic Transmission/Transaxle Diagnosis. If multiple DTCs are stored, diagnose each DTC according to the following priority:
^ PCM Error DTCs
^ System Voltage DTCs
^ System Reference 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
5. A scan tool parameter which is not within the typical range may help to isolate the area which is causing the problem.