- Powertrain/On-Board Diagnostic System Check

Flow Of Diagnosis:

Data Link Circuit:

Circuit Description:
The On-Board Diagnostic (OBD) system check is an organized approach to identifying a problem created by an electronic engine 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 MIL. The MIL should be "ON" steady.

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

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

4. Most of the diagnostic charts use a 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 or 51, but not transmit serial data. It is necessary to have both B+ and ground available at the PCM for serial data to be established.

5. Although the PCM is powered up, a "Cranks But Will Not Run" symptom could exist because of an PCM or system problem.

6. This step will isolate if the customer complaint is a MIL or a driveability problem with no MIL. Refer to "PCM Diagnostic Trouble Codes Chart" Testing and Inspection
in this section for a list of valid DTC(s). An invalid DTC may be the result of a faulty scan tool, EPROM or PCM.

7. Comparison of actual control system data with the typical values is a quick check to determine if any parameter is not within limits. Keep in mind that a base engine problem (i.e. advanced cam timing) may substantially alter sensor values.

8. If the actual data is not within the typical values established, the C-Charts will provide a functional check of the suspect component of system.