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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 181 (ENGINE CONTROLS - SELF-DIAGNOSTICS - 2.2L)DIAGNOSTIC TESTSDTC P0420: CATALYST SYSTEM EFFICIENCY BELOW THRESHOLDDIAGNOSTIC 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 2003 Saturn Vue. 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. Observe the DTC information on the scan tool. Are there any other component DTCs set? If yes, go to DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, go to next step.
  3. Allow engine to reach operating temperature. Ensure closed-loop operation is achieved. Increase the engine speed to 1500 RPM for one minute. Monitor the HO2S 1 and the HO2S 2 voltage parameters with a scan tool. Is the HO2S 2 voltage parameter as active as the HO2S 1 voltage parameter? If yes, go to step  5. If no, go to next step.
  4. Clear the DTCs with a scan tool. Start engine. Operate the vehicle within the Conditions for Running the DTC. See CONDITIONS FOR RUNNING DTC . You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records. Did DTC P0420 set? If yes, go to next step. If no, go to DIAGNOSTIC AIDS .
    5. NOTE: Verify that the TWC converter is a high quality part that meets the OEM specifications.
  5. Visually and physically inspect the three-way catalytic converter for dents or damage, severe discoloration caused by excessive temperatures, holes or punctures, or internal rattles caused by a damaged catalyst element. Did you find a condition? If yes, go to step  9. If no, go to next step.
  6. Visually inspect the exhaust system for leaks, physical damage, loose or missing hardware, or HO2S 2 for proper torque. Did you find and correct the condition? If yes, go to step  10. If no, go to next step.
  7. Visually inspect the catalyst monitor HO2S 2 for pigtail and wiring harness making contact with the exhaust or any good ground, or road damage. Did you find a condition? If yes, go to step  8. If no, go to step  9.
  8. Replace HO2S 2. After replacing sensor, go to step  10.
    9. NOTE: Locate and repair the cause of the TWC converter failure before installing the replacement converter.
  9. After replacing TWC converter, go to next step.
  10. Using scan tool, clear DTCs. Turn ignition off for 30 seconds. Operate vehicle within the conditions for running the DTC. See CONDITIONS FOR RUNNING DTC . You may also operate vehicle within the conditions observed from the Freeze Frame/Failure Records. Did the DTC fail this ignition? If yes, go to step  2. If no, go to next step.
  11. 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.
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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.