Basic Diagnostic Procedure
NOTE:
Most engine control problems are the result of mechanical breakdowns, poor electrical connections or damaged vacuum hoses. Before considering the computer system as a possible cause of problems, ignition high tension wires, fuel supply, electrical connections and vacuum hoses should be checked. Failure to do so may result in lost diagnostic time.
Diagnosis of the computerized engine control system should be performed in the following order:
- Make sure that all engine systems not related to the computer system are operating properly. Do not proceed with testing unless all other problems have been repaired.
- Perform appropriate DIAGNOSTIC CIRCUIT CHECK for that system. If codes were displayed (other than Code 12), decide whether they are "hard" or "intermittent" codes. "Hard" codes will cause the "SERVICE ENGINE SOON" light to illuminate continuously while engine is running. See ECM TROUBLE CODE DEFINITION table in this article.
- If no trouble codes were displayed, proceed to appropriate SYSTEM PERFORMANCE CHECK.
- If no trouble is indicated by the SYSTEM PERFORMANCE CHECK and/or a driveability problem exists, refer to SYMPTOM DIAGNOSIS in the CCC TESTS W/O CODES article in this section and/or the SCAN TESTER USAGE procedures in this article. The comments there will send you to the proper component charts or tell you the most likely system/component to check.
- After any repairs are made, clear any trouble codes and perform SYSTEM PERFORMANCE CHECK or FIELD SERVICE MODE check.