Diagnostic Circuit Check

(OBD) System Check Using Tech 1 Scan Tool:

OBD Diagnostic Diagram (PCM):

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 "Service engine Soon" light. The "SES" light should be "ON" steady.

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

3. This test cheeks the ability of the PCM to control the "SES" light. With the diagnostic terminal grounded, the "SES" light should flash a Code 12 three times, followed by any trouble code stored in memory. Depending upon the type of PCM, PROM or MEM-CAL error may result in the inability to flash Code 12.

4. This test check "Quad-Driver" circuit. If the "SES" light flashes less than three times, indicates that there is a problem with the "SES" light circuit or the PCM control of that circuit.

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

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

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

8. Comparison of actual control system data with the Typical Tech 1 Data 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.

9. Installation of a Tech 1 "Scan" tool will provide a good ground path for the PCM and may hide a driveability complaint due to poor PCM grounds.

10. If the actual data is not within the typical values established, refer to the applicable diagnosis section to provide a functional check for the suspect component or system.