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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 51 (ENGINE CONTROL SYSTEM - TESTS W/CODES - INTRODUCTION)SELF-DIAGNOSTIC SYSTEMON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK4.0L
1999 GMC Forward Control 5.7 R, Automatic

On-Board Diagnostic (OBD) System Check: 4.0L

1999 GMC Forward Control 5.7 R, AutomaticSECTION 4.0L
WARNING: This page does not describe the selected car, but rather 68 other vehicles, including the 1999 Saturn SW2, 1999 Saturn SW1, 1999 Saturn SL2, 1999 Saturn SL1, and 1999 Saturn SL. However, it is still accessible from the selected car via links, so may be relevant.
  1. Check for one or more of the following conditions:
    • MIL illuminates with engine running.
    • Performance/driveability concern.
    • Suspected fault in powertrain component or system.
    If one or more of the listed conditions are present, go to next step. If none of the listed conditions are present, system is okay.
  2. Ensure battery is fully charged. Turn ignition off. Connect scan tool. Turn ignition on. If scan tool displays PCM data, go to next step. If scan tool does not display PCM data, go to DLC DIAGNOSIS OR NO SCAN TOOL DATA .
  3. Attempt to start engine. If engine starts and continues to run, go to next step. If engine does not start, or starts and stalls, go to NO START DIAGNOSIS in appropriate BASIC DIAGNOSTIC PROCEDURES article.
  4. Using scan tool, observe DTC status for MIL REQUEST, FAIL THIS IGN, LAST TEST FAIL and HISTORY. If any DTCs are stored, save freeze frame and failure records information using scan tool CAPTURE INFO feature. Diagnose DTCs. See DIAGNOSTIC TESTS in appropriate TESTS W/CODES article. If DTCs are not present, go to next step.
  5. Compare PCM data values displayed on scan tool with typical control system data values. If values are within limits, system is okay. If values are not within limits, see appropriate SYSTEM & COMPONENT TESTING article.
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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.