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Home >> Chevrolet >> 2002 >> Blazer 4D Utility, 4WD, Part Time >> Repair and Diagnosis >> External Pages >> Different car >> Section 379 (Engine Controls Self-Diagnostics - 3.4L) >> Diagnostic Tests >> DTC P0420: TWC System - Low Efficiency >> Diagnostic Procedures

Diagnostic Procedures

WARNING: This page is about a different car, the 2003 Pontiac Grand Am, 2003 Oldsmobile Alero, 2003 Chevrolet Monte Carlo, and 2003 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 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 may not run.
  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. Fully open the hood. Operate the vehicle within the Conditions For Running. See CONDITIONS FOR RUNNING . The test may need to be completed up to 6 times in order to pass or fail. If more than 6 tests have been attempted and the DTC has not passed or failed this ignition cycle, the test may be aborting. Does the scan tool indicate the DTC ran and passed this ignition cycle? If yes, system is okay at this time. If no, go to next step.
  4. Visually and physically inspect the TWC for damage. See RESTRICTED EXHAUST under EXHAUST SYSTEMS in SYSTEM & COMPONENT TESTING - 3.4L ALERO, GRAND AM, IMPALA & MONTE CARLO article. Check for dents, severe discoloration caused by excessive temperatures, holes and internal rattle caused by damaged catalyst substrate. Verify that the TWC is a proper Original Equipment Manufacturer (OEM) part. Did your inspection reveal a problem? If yes, go to step  9. If no, go to next step.
  5. Visually and physically inspect the exhaust system for leaks. See EXHAUST LEAKAGE under EXHAUST SYSTEMS in SYSTEM & COMPONENT TESTING - 3.4L ALERO, GRAND AM, IMPALA & MONTE CARLO article. Did your inspection reveal a problem? If yes, go to step  7. If no, go to next step.
  6. Visually inspect the oxygen sensors and the oxygen sensor wiring harnesses. Did your inspection reveal a condition? 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 the HO2S sensor wiring 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 3-way Catalytic Converter (TWC). After repair, go to step  10.
    10. NOTE: If the A/C is left ON, the following diagnostic may not run.
  10. Clear the 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. See CONDITIONS FOR RUNNING . The test may need to be completed up to 6 times in order to pass or fail. If more than 6 tests have been attempted and the DTC has not passed or failed this ignition cycle, the test may be aborting. See DIAGNOSTIC AIDS . 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. Are there any additional DTCs that you have not diagnosed? If yes, go to DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, system is okay at this time.
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