Diagnostic System Check

Circuit Description
The On-Board Diagnostic (OBD) System Check is an organized approach to identifying a problem created by a control module system malfunction. The OBD is the starting point for any driveability concern diagnosis. The system check directs the service technician to the next logical step in diagnosing the concern.

Perform this check only if there is a driveability complaint or if you have been sent to this system check from another section in the service information.

Understanding the table properly reduces the diagnostic time. Proper use of the OBD helps prevent the unnecessary replacement of good pans.

Diagnostic Aids

Important: Do not clear the DTCs unless directed by a diagnostic procedure. Clearing the DTCs will also clear valuable Freeze Frame and Failure Records data.

Inspect all related wiring and connections including the connections at the VCM. These may cause an intermittent malfunction.

Any base engine problem could alter the scan tool data. Examples of these are: distributor timing, valve timing, and intake gasket leaks.

An intermittent may be caused by any of the following conditions:
^ A poor connection.
^ Rubbed through wire insulation.
^ A broken wire inside the insulation.

Thoroughly check any circuitry that is suspected of causing the intermittent complaint. Refer to Intermittents and Poor Connections Diagnosis in Symptoms.

Test Description
The numbers below refer to the step numbers in the diagnostic table.
1. This step determines if the scan tool powers up when connected to the DLC.
2. The MIL should be ON when you turn ON the ignition.
4. Refer to DTC List (DTC Type Table) for a list of valid DTCs.
5. Refer to Data Link Connector (DLC) Schematics in order to determine all of the modules communicating on the Class 2 Serial Data circuit. If any of the modules are not present in the Diagnostic Circuit Check, then the modules never established communications. The scan tool will indicate a YES state for communicating modules and a NO state for modules that had established yet no longer communicate.
7. If the vehicle does not start or starts and stalls, the Passlock system could be the cause.
8. This test determines if the MIL can be controlled by the VCM.
9. Comparing the ACTUAL and TYPICAL scan tool data values will aid in determining whether an engine parameter is within limits (i.e., Advanced timing can substantially alter sensor values).