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Home >> Chevrolet >> 2002 >> Blazer 4D Utility, 4WD, Part Time >> Repair and Diagnosis >> External Pages >> Different car >> Section 370 (Engine Control System - Self-Diagnostics - 4.2L) >> Diagnostic Tests >> DTC P0420: Catalyst System Efficiency Below Threshold (Bank 1) >> Diagnostic Procedures

Diagnostic Procedures

WARNING: This page is about a different car, the 2003 Oldsmobile Bravada, 2003 GMC Envoy XL, 2003 GMC Envoy, and 2003 Chevrolet TrailBlazer. However, it is still accessible from the selected car via links, so may be relevant.
  1. Did you perform the Diagnostic System Check - Engine Controls? If yes, go to next step. If no, see DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM.
  2. Start and operate engine at idle until Closed Loop is achieved. Increase engine speed to 1500 RPM. Monitor the HO2S 1 and 2 voltage parameters on scan tool. Is the HO2S 2 voltage parameter as active as the HO2S 1 voltage parameter? If yes, go to step  4. If no, go to next step.
  3. Observe the Freeze/Frame Records for this DTC. Clear the DTCs with a scan tool. Turn the OFF all accessories. Operate the vehicle within the Conditions For Running this DTC. See CONDITIONS FOR RUNNING DTC . 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 DTC P0420 ran and passed this ignition? If yes, go to DIAGNOSTIC AIDS . If no, go to next step.
    4. NOTE: Verify that the TWC is a high quality part that meets OEM specifications.
  4. Visually and physically inspect the catalytic converter for dents, severe discoloration caused by excessive temperatures, road damage, internal rattle caused by damaged catalyst substrate, or restrictions. Did you find a condition? If yes, go to step  8. If no, go to next step.
  5. 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  9. If no, go to next step.
  6. Visually inspect the HO2S 2 pigtail and wiring harness contacting the exhaust or a ground. Inspect for physical damage. Did you find a condition? If yes, go to next step. If no, go to step  8.
  7. Replace HO2S 2 sensor. After replacing sensor, go to step  9.
    8. NOTE: In order to avoid damaging the replacement TWC converter, correct the engine misfire or mechanical fault before replacing converter.
  8. Replace catalytic converter. After replacing converter, go to next step.
  9. Clear the DTCs with a scan tool. Turn ignition off for 30 seconds. Start engine. Operate 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 the DTC fail this ignition? If yes, go to step  2. If no, go to next step.
  10. Observe the Capture Info with a scan tool. Are there any DTCs that you have not diagnosed? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, system is okay.
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