On-Board Diagnostic System Check (OBD)

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 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.
2. Is there a Diagnostic Trouble Code (DTC) stored? If a DTC is stored, go directly to the numbered DTC chart. This will determine if the fault is still present. If no DTC is stored, proceed to the next step.
3. Scan serial data transmitted by the Powertrain Control Module (PCM). This involves reading the information available on the serial data stream with one of the tools available for that purpose. 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.