Diagnostic Circuit Check

Diagnostic Circuit Check:

OBD Circuit:

NOTE:
- This is the starting point for all diagnostic procedures or finding the cause of an emissions test failure. Always begin here and always follow the flow chart as directed.
- Numbers below refer to circled numbers in flow chart.

1. Get an accurate description of the complaint and verify. Include filling out the DRIVEABILITY WORKSHEET and VEHICLE REPAIR HISTORY. Refer to INFO TYPE/TECHNICAL MANAGEMENT SUPPORT for forms.

Perform Underhood Inspection:
- Check vacuum hoses for splits, kinks, and proper routing.
- Check ignition wires for cracking, hardness, and proper connections at the distributor and spark plugs.
- Check all wiring for proper connections, pinches, and cuts.
- Check wiring harness for proper routing.
- Check for missing components.

2. Check TSB's for:
- Manual Updates
- Testing Procedures
- Warranty
- Recall information.

3. Refer to PROM - Programmable Read Only Memory.

4. This step is a check for the proper operation of the MIL (Service Engine Soon). The MIL should be "ON" steady. If there is no "SERVICE ENGINE SOON" light, refer to DIAGNOSTIC CHARTS/CHART A-1
Related Tests, Information and Procedures

5. Indicates Circuit 451, "B" terminal, is grounded. Repair grounded circuit.

6. Use TECH 1 scan tool to aid diagnosis, therefore, serial data must be available. If a PROM (MEM-CAL) error is present, the PCM may have been able to flash DTC 12/51, but not enable serial data

7. Although the PCM is powered up, a "Cranks But Will Not Run" symptom could exist because of a PCM or system problem. If "NO", refer to CHART A-3 ENGINE CRANKS BUT WILL NOT RUN
Related Tests, Information and Procedures

8. This step will isolate if the customer complaint is a MIL or a driveability problem with no MIL. For a list of valid DTCs, refer to DIAGNOSTIC TROUBLE CODE DESCRIPTION
Diagnostic Trouble Code Descriptions
If "YES", proceed to the applicable DTC chart. Refer to TROUBLE CODE CHARTS
Testing and Inspection
An invalid DTC may be the result of a faulty scan tool, PROM (MEM-CAL) or PCM.

9. 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. Refer to SCAN-TOOL DIAGNOSTICS/TYPICAL SCAN DATA VALUES
Scan Tool Data List

DIAGNOSTIC AIDS:
If all systems appear to be functioning within normal ranges, refer to DIAGNOSIS BY SYMPTOM
Testing and Inspection