On-Board Diagnostic (OBD) System Check: Description

The On-Board Diagnostic (OBD) System Check is the starting point for any driveability complaint diagnosis. Before any Diagnostic Trouble Codes (DTCs) are accessed, perform a visual underhood inspection checking battery and grounds for clean and tight connections. Check for poor connections or damage to wiring harness and electrical connectors. Check for corroded terminals or corrosion build-up on any connection. Repair as necessary. The OBD system check is an organized approach to identifying problems created by an electronic engine control system malfunction.

1. Turn ignition on with engine off. Verify Malfunction Indicator Light (MIL) is on. If MIL is on, go to next step. If MIL is not on, go to NO MALFUNCTION INDICATOR LIGHT.
2. Turn ignition off. Install scan tool. Turn ignition on. If scan tool displays PCM data, go to next step. If scan tool does not display PCM data, verify scan tool is properly installed. If scan tool is properly installed, go to MALFUNCTION INDICATOR LIGHT ON STEADY  .
3. Using scan tool, command MIL OFF. If MIL turns off, go to next step. If MIL does not turn off, go to MALFUNCTION INDICATOR LIGHT ON STEADY.
4. Using scan tool, check if any DTCs have been set. If no DTCs are set, go to next step. If any DTCs are set, use scan tool to store Freeze Frame Data and Failures Record Data. See appropriate TESTS W/CODES article.
5. Start engine. If engine starts and continues to run, go to next step. If engine will not run, go to ENGINE CRANKS BUT WILL NOT RUN.
6. Turn ignition on with engine off. Check ECT and TP sensors for proper operation. See SYSTEM/COMPONENT TESTS article. If sensors are operating properly, go to next step. If sensors are not operating properly, replace as necessary.
7. Start engine. Allow engine to reach 194-203°F (90-95°C). While engine is reaching operating temperature, check ECT sensor, MAP sensor, O2S sensor and IAC valve for proper operation. See SYSTEM/COMPONENT TESTS article. If sensors are operating properly, go to next step. If sensors are not operating properly, replace as necessary.
8. Ensure scan tool values displayed for ECT, MAP and O2S sensors are correct. See appropriate SCAN TOOL VALUES DATA table under PIN VOLTAGE (VALUE) TESTS in PIN VOLTAGE CHARTS article. If values are correct, OBD system check is complete. If driveability problem still exists, go to TESTS W/O CODES article. If values are not correct, go to SYSTEM/COMPONENT TESTS article.