The On-Board Diagnostic (OBD) System Check is an organized approach to identifying a problem that is 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 order to diagnose the problem.

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 to prevent the unnecessary replacement of good parts.

Important: Do not clear the DTCs unless you are directed to do so by a diagnostic procedure. Clearing the DTCs will also clear valuable Freeze Frame/Failure Records data.

Inspect all of the related wiring and inspect all of the connections, including the connections at the vehicle control module (VCM) . These connections may cause an intermittent malfunction.

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

An intermittent may be caused by any of the following conditions:

Thoroughly inspect any circuitry that is suspected of causing the intermittent complaint. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.

If a repair is necessary, refer to Wiring Repairs or Connector Repairs in Wiring Systems.

The numbers below refer to the step numbers in the diagnostic table.

1. This step determines if the scan tool powers up when it is connected to the data link connector (DLC).

2. The malfunction indicator lamp (MIL) should be ON when you turn ON the ignition.

4. Refer to Diagnostic Trouble Code (DTC) List/Type for a list of valid DTCs.

5. Refer to Data Link Connector Schematics in Data Link Communications in order to determine all of the modules that are 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 will indicate a NO state for modules that had established communications but are no longer communicating.

7. If the vehicle does not start or if the vehicle 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 scan tool data values to the TYPICAL scan tool data values will help to determine whether an engine parameter is within limits. For example, advanced timing can substantially alter sensor values.