Chart 1 On-Board Diagnostic System Check

Chart I - On Board Diagnostic System Check:

Schematic Diagram:

CHART I

DESCRIPTION
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.
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. Expected readings under a particular operating condition can be found at the end of the PCM charts.



ON-BOARD DIAGNOSTIC SYSTEM CHECK (OBD SYSTEM CHECK)
The on-board diagnostic system check (OBD system check) is always the starting point for any malfunction diagnostic trouble code or driveability problem. This check verifies the PCM's on-board diagnostics are functioning properly. While performing an on-board diagnostic system check (OBD system check) record diagnostic trouble code(s) and information flags for use in diagnosing possible sub-system failures.

Key On - Engine off - Service Engine Soon (MIL) telltale should be on steady.

When DLC connector Pin B is grounded to Pin A with Key On - Engine Off diagnostic mode is entered.

DTC 12 will flash indicating diagnostic communication.

DTC 12 will flash three times followed by any other codes that may be stored.

The following circuits are energized when the vehicle enters diagnostic mode:
^ Cooling fan is turned on.
^ IAC valve is pulsed in and out.
^ EVO is cycled.
^ EVAP Canister purge is cycled on.
^ EGR is cycled on.
^ Hot telltale turned on.