Vehicle On-Board Diagnostic System Check

VEHICLE ON-BOARD DIAGNOSTIC SYSTEM CHECK
After the visual/physical underhood inspection, the vehicle on-board diagnostic (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 vehicle On-Board Diagnostic (OBD) system. Since this is the starting point for the diagnostic procedures or finding the cause of an emissions test failure, always begin here.
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 Step 3.
3. Scan serial data transmitted by the PCM. This involves reading the 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 section. Expected readings under a particular operating condition can be found in the Scan tool data definition section.