On-Board Diagnostic (OBD) System Check

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

1. Check for one or more of the following conditions:
   • Malfunction Indicator Light (MIL) illuminated with engine running.
   • Confirmed customer driveability concern.
   • Suspected fault in computerized engine control system.

   If any conditions are present, go to next step. If specified conditions are not present, system is okay at this time. Check for an intermittent problem. See INTERMITTENTS in appropriate TROUBLE SHOOTING - NO CODES article.

2. Turn ignition on with engine off. Observe MIL. If MIL illuminates, go to next step. If MIL does not illuminate, go to NO MALFUNCTION INDICATOR LIGHT (MIL)  under CIRCUIT TESTS.
3. Turn ignition off. Install scan tool according to manufacturer's instructions. Turn ignition on with engine off. If scan tool displays PCM data, go to next step. If scan tool does not display PCM data, go to DATA LINK CONNECTOR (DLC) DIAGNOSIS  under CIRCUIT TESTS.
4. Attempt to start engine. If engine starts and runs, go to next step. If engine will not run, go to appropriate NO START-ENGINE CRANKS OKAY under NO START DIAGNOSIS in appropriate BASIC DIAGNOSTIC PROCEDURES article.
5. Observe DTC data. If no DTCs are set, go to next step. If any DTCs are set, save freeze frame data using scan tool, and go to appropriate Diagnostic Trouble Code (DTC). See DIAGNOSTIC TESTS .
6. If MIL is illuminated and no DTCs are set, diagnose using DTC P1641. See DIAGNOSTIC TESTS . If MIL is off and no DTCs are set, go to next step.
7. Compare scan tool data with values shown in SCAN TOOL DATA VALUES  table under SCAN TOOL DATA. If displayed values are within specified ranges, suspect an intermittent fault. See INTERMITTENTS in appropriate TROUBLE SHOOTING - NO CODES article. If displayed values are not within specified ranges, diagnose suspect component. See appropriate SYSTEM & COMPONENT TESTING article.