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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 89 (ENGINE CONTROLS - SELF-DIAGNOSTICS - 4.0L)DIAGNOSTIC TESTSDTC P0420: CATALYST SYSTEM LOW EFFICIENCYDIAGNOSTIC PROCEDURES
1999 GMC Forward Control 5.7 R, Automatic

Diagnostic Procedures

1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedures
WARNING: This page is about a different car, the 2002 Oldsmobile Aurora. However, it is still accessible from the selected car via links, so may be relevant.
  1. Did you perform the Diagnostic System Check - Engine Controls? If yes, go to next step. If no, go to DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM.
    2. NOTE: If any other DTCs are set, diagnose the other DTCs first.
  2. Are any other DTCs set? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, go to next step.
  3. Inspect the exhaust system for leaks, for damage, and for loose or missing hardware. If a condition is found, repair the condition as necessary. Did you find and correct the condition? If yes, go to step  6. If no, go to next step.
  4. Inspect for the following conditions:
    • The correct original equipment catalytic converter, correctly installed.
    • Converter damage such as dents, severe discoloration, or holes.
    • Properly installed oxygen sensors.
    • Damaged wiring, or improper wiring connections.

    If a condition is found, repair the condition as necessary. Did you find and correct the condition? If yes, go to step  6. If no, go to next step.

  5. Replace the catalytic converter. After repairs, go to next step.
  6. Use the scan tool in order to clear the DTCs. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions for Running DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step  2.
  7. With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, system is okay.
RENDER: 1.0x

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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.