- Overview of Diagnostic Steps

After the visual/physical underhood inspection, the on-board diagnostic system check (OBD system check) is the starting point for all diagnostic procedures or finding the cause of an emissions test failure.

The correct procedure to diagnose a problem is to follow three basic steps:
1. Are the on-vehicle diagnostics working? This is determined by performing the on-board diagnostic system check (OBD system check). Since this is the starting point for the diagnostic procedures or finding the cause of an emissions test failure, always begin here. Testing and Inspection
2. Is there a diagnostic trouble code stored? If a diagnostic trouble code is stored, go directly to the numbered diagnostic trouble code chart. This will determine if the fault is still present. If no diagnostic trouble code is stored, proceed to the next Step.
3. Scan serial data transmitted by the PCM. This involves reading the information available on the serial data stream with one of the tools available for that purpose. Information on these tools and the meaning of the various displays can be found in the succeeding paragraphs.