- Overview

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). This is the starting point for the diagnostic procedures or finding the cause of an emissions test failure.
2. Is there a diagnostic trouble code stored? If a diagnostic trouble code is stored, go directly to the numbered diagnostic trouble code chart. Diagnostic Trouble Code Descriptions
The chart will indicate if the fault is still present. If no diagnostic trouble code is stored, proceed to Step 3.
3. Scan serial data transmitted by the ECM. This involves reading information available on the serial data stream with a Scan tool. The meaning of the various displays can be found in the Scan tool data definition. Expected readings under a particular operating condition can be found in the Scan tool data definition. Symptom Related Diagnostic Procedures