- Powertrain/On-Board Diagnostic System Check

Diagnostic Chart:

OBD System Schematic:

CIRCUIT DESCRIPTION
The On-Board Diagnostic System Check is an organized approach to identifying a problem created by an electronic engine control system malfunction. It must be the starting point for any driveability complaint diagnosis, because it directs the service technician to the next logical step in diagnosing the complaint. Understanding the chart and using it correctly will reduce diagnostic time and prevent the unnecessary replacement of good parts.

TEST DESCRIPTION
Numbers below refer to circled numbers on the diagnostic chart.

1. Checks the Malfunction Indicator Lamp (MIL) operation.
2. Checks to see if the Engine Control Module (ECM) self-diagnostic mode is operating.
3. Checks to see if the ECM's serial data output is operating.
4. Checks to see if vehicle will start.
5. Checks for any Diagnostic Trouble Codes (DTCs) that are stored in the ECM's memory with the engine running.
6. Checks for any DTCs that are stored in the ECM's memory with the engine off.
7. Compares ECM's control data to typical data values.
8. Checks to see if codes are intermittent problems.

DIAGNOSTIC AIDS
When the diagnostic terminal in either the Duty Check Data Link Connector (DLC) or the DIAG SW in the junction block are grounded with the ignition switch in the "ON" position (engine not running), the system enters what is called the diagnostic mode. In this mode, the ECM will perform the following functions:

1. Display Diagnostic Trouble Code 12 by flashing the Malfunction Indicator Lamp (indicating the system is operating correctly.)
2. Display any stored DTCs by flashing the MIL. If DTCs other than DTC 12 are stored, DTC 12 will not be flashed.
3. Output idle speed control duty through the duty check terminal of the Duty Check DLC.