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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 239 (ENGINE CONTROLS SELF-DIAGNOSTICS - 3.1L ENGINE)DIAGNOSTIC TESTSDTC P0420: CATALYST SYSTEM - LOW EFFICIENCYDIAGNOSTIC PROCEDURE
1999 GMC Forward Control 5.7 R, Automatic

Diagnostic Procedure

1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedure
WARNING: This page is about a different car, the 2003 Pontiac Grand Prix, 2003 Chevrolet Malibu, and 2003 Buick Century. However, it is still accessible from the selected car via links, so may be relevant.
  1. Perform diagnostic system check - engine controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check - engine controls, go to next step.
  2. Are any other DTCs set? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, go to next step.
    3. NOTE: If the A/C is left ON the diagnostic may not run.
  3. Turn the A/C OFF. Start the engine and wait until the vehicle is fully warmed up at least 10 minutes, then above 1500 RPM for 5 additional minutes. (One minute for each additional attempt at 1500 RPM on same key cycle). The vehicle may be driven for the 5 minute run time. Fully open the hood. Operate the vehicle within the Conditions For Running Test as specified in the supporting text. See CONDITIONS FOR RUNNING TEST . If more than 6 tests have been attempted and the DTC has not passed or failed this ignition cycle the test may be aborted. See DIAGNOSTIC AIDS . The test may need to be completed up to 6 times to pass or fail. Does the DTC reset? If yes, go to next step. If no, system is okay at this time.
  4. Check the 3-way catalytic converter for the following damage:
    • Dents.
    • Severe discoloration caused by excessive temperatures.
    • Holes.
    • Internal rattle caused by damaged catalyst substrate.

    Verify that the 3-way catalytic converter is a proper original equipment manufacturer part. Did your inspection reveal a condition? If yes, go to step  9. If no, go to next step.

  5. Check the exhaust system for leaks. See EXHAUST LEAKAGE under EMISSION SYSTEMS & SUB-SYSTEMS in SYSTEM & COMPONENT TESTING - 3.1L CENTURY, GRAND PRIX & MALIBU article. Check the exhaust system for restrictions. See EXHAUST RESTRICTION under EMISSION SYSTEMS & SUB-SYSTEMS in SYSTEM & COMPONENT TESTING - 3.1L CENTURY, GRAND PRIX & MALIBU article. Did you find any of the conditions? If yes, go to step  7. If no, go to next step.
  6. Check the oxygen sensors and the oxygen sensor wiring harness for damage. Did you find any of the conditions? If yes, go to step  8. If no, go to step  9.
  7. Repair the exhaust system as necessary. After repair, go to step  10.
  8. Repair as necessary. After repair, go to step  10.
    9. CAUTION: A misfiring condition may damage the replacement three-way catalytic converter. Check for a misfiring condition. If a misfire is present, repair the misfiring condition before replacing the three-way catalytic converter.
  9. Replace the three way catalytic converter. After repair, go to next step.
    10. NOTE: If the A/C is left ON the following diagnostic may not run.
  10. Clear the DTCs using a scan tool. Turn the A/C OFF. Start the engine. Allow the engine to warm for at least ten minutes, then above 1500 RPM for 5 additional minutes. (One minute for each additional attempt at 1500 RPM on same key cycle). The vehicle may be driven for the 5 minute run time. Fully open hood. Operate the vehicle within the Conditions For Running Test, as specified in the supporting text. See CONDITIONS FOR RUNNING TEST . If more than 6 tests have been attempted and the DTC has not passed or failed this ignition cycle the test may be aborted. See DIAGNOSTIC AIDS . Test may need to be completed up to 6 times to pass or fail. Does the DTC reset? If yes, go to step  4. If no, go to next step.
  11. Using the scan tool, observe the stored information in 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 at this time.
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.