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On-Board Diagnostic (OBD) System Check

NOTE: Do not perform this check if a driveability complaint is not present.

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 next logical step in diagnosing a complaint. Understanding and using it correctly will reduce diagnostic time and prevent unnecessary replacement of good parts.

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 TESTS W/O CODES article.

  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.
  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 NO START - ENGINE CRANKS OKAY under NO START DIAGNOSIS in BASIC TESTING 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 Diagnostic Trouble Code (DTC). See DIAGNOSTIC TROUBLE CODES (DTCS)  .
  6. If MIL is illuminated and no DTCs are set, go to MALFUNCTION INDICATOR LIGHT (MIL) ON STEADY under CIRCUIT 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 TESTS W/O CODES article. If displayed values are not within specified ranges, see SYSTEM/COMPONENT TESTS article.
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