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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 79 (ENGINE CONTROL SYSTEM SELF-DIAGNOSTICS - 3.4L)DIAGNOSTIC TESTSDTC P0420: TWC SYSTEM LOW EFFICIENCY (BANK 1)DIAGNOSTIC 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 Pontiac Grand Am, 2002 Oldsmobile Alero, 2002 Chevrolet Monte Carlo, and 2002 Chevrolet Impala. 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 there any other DTCs set? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, go to next step.
    3. NOTE: If A/C is left ON the following diagnostic may not run. If more than 6 tests have been attempted and DTC has not passed or failed this ignition cycle the test may be aborting.
  3. Clear DTCs using the scan tool. Turn the A/C OFF. Start the engine and wait until the vehicle is fully warmed up. Fully open the hood. Operate the vehicle within the Conditions for Running this DTC. See CONDITIONS FOR RUNNING DTC . Test may need to be completed up to 6 times in order to pass or fail. Does the scan tool indicate the DTC ran and passed this ignition cycle? If yes, system okay. If no, go to next step.
  4. Inspect for dents, severe discoloration caused by excessive temperatures, holes, or internal rattle caused by damaged catalyst substrate. Ensure that the TWC is a correct original equipment manufacturer part. Did your inspection reveal a problem? If yes, go to step  9. If no, go to next step.
  5. Inspect the exhaust system for leaks. Did your inspection reveal a problem? If yes, go to step  7. If no, go to next step.
  6. Inspect oxygen sensors for damage or oxygen sensor wiring harness for damage. Was a repair necessary? If yes, go to step  8. If no, go to step  9.
  7. Repair exhaust system as necessary. After repairs, go to step  10.
  8. Repair HO2S and wiring as necessary. After repairs, go to step  10.
    9. NOTE: 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 catalytic converter. After repairs, go to next step.
    10. NOTE: If A/C is left ON, the following diagnostic may not run. If more than 6 tests have been attempted and DTC has not passed or failed this ignition cycle the test may be aborting.
  10. Clear DTCs using the scan tool. Turn the A/C OFF. Start the engine and wait until the vehicle is fully warmed up. Fully open hood. Operate the vehicle within the Conditions for Running this DTC. See CONDITIONS FOR RUNNING DTC . Test may need to be completed up to 6 times in order to pass or fail. Does the scan tool indicate the DTC ran and passed this ignition cycle? If yes, go to next step. If no, go to step  4.
  11. Using scan tool, observe stored information and 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.