Diagnostic Circuit Check - With Diagnostic Scan Tool

OBD System Check:

OBD Diagnostic Circuit:

CIRCUIT DESCRIPTION
The diagnostic circuit check is an organized approach to identifying a problem created by a Computer Command Control System malfunction. It must be the starting point for any driveability complaint diagnosis, because it directs the service technician to the next logical step in diagnosing the complaint. Understanding the chart and using it correctly will reduce diagnostic time and prevent the unnecessary replacement of good parts.

TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.

1. This step is a check for the proper operation of the MIL "Service Engine Soon" light. The MIL light should be "ON" steady.

2. No MIL light at this point indicates that there is a problem with the MIL light circuit or the PCM control of that circuit.

3. This test cheeks the ability of the PCM to control the MIL light. With the diagnostic terminal grounded, the MIL light should flash a DTC 12 three times, followed by any trouble code's stored in memory. A PCM or PROM error may result in the inability to flash DTC 12.


4. Use Tech 1 to aid diagnosis, therefore, serial data must be available. If a PROM error is present, the PCM may have been able to flash DTC 12/51, but not enable serial data.


5. This step will isolate if the customer complaint is a MIL light or a driveability problem with no MIL light. Refer to "Diagnostic Code Identification" for a list of valid codes. An invalid code may be the result of a faulty "Scan" tool, PROM or PCM.