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Home >> Buick >> 2002 >> Regal LS >> Repair and Diagnosis >> Engine Performance >> System >> Engine Control System Self-Diagnostics - 3.8L >> Diagnostic Tests >> DTC P0420: Catalyst System Low Efficiency >> Diagnostic Procedures
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Diagnostic Procedures

  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 following diagnostic will not run. 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 .
  3. Clear the DTCs using the scan tool. Turn the A/C off. Start the engine and wait until the vehicle is fully warmed up, at least 10 minutes. Next, idle the engine above 1500 RPM for an additional 5 minutes. The vehicle may be driven for the 5 minute run time. Idle for 1 minute for each additional attempt at 1500 RPM on same key cycle. Fully open the hood. Operate the vehicle within the 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 is okay. If no, go to next step.
  4. Visually and physically inspect the three-way catalytic converter for damage. Check for the following conditions:
    • Dents.
    • Severe discoloration caused by excessive temperatures.
    • Holes.
    • Internal rattle caused by damaged catalyst substrate.

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

  5. Visually and physically inspect the exhaust system for leaks, restrictions, damage. Repair condition as necessary. Did your inspection reveal a condition? If yes, go to step  7. If no, go to next step.
  6. Visually check the Heated Oxygen Sensors (HO2S), and the HO2S wiring harness for damage. Did you complete the action? If yes, go to step  8. If no, go to step  9.
  7. Repair exhaust system. After repairs, go to step  10.
  8. Replace the HO2S sensor or repair 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 TWC. Check for possible engine misfire DTC or engine mechanical problem. After repairs, go to next step.
    10. NOTE: If the A/C is left on, the following diagnostic will not run. 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 .
  10. Clear DTCs using the scan tool. Turn the A/C off. Start the engine and wait until the vehicle is fully warmed up, at least 10 minutes. Idle the engine above 1500 RPM for 5 additional minutes. The vehicle may be driven for the 5 minute run time. Idle the engine 1 minute for each additional attempt at 1500 RPM on same key cycle. Fully open the hood. Operate the vehicle within the 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. Does scan tool displays any DTCs that have not been diagnosed? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, system is okay.
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