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HOMESERVICE MANUALSCHEVROLET2008CHEVY EXPRESS G2500, VAN CARGO, 6.6 6REPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 28 (ENGINE CONTROL SYSTEM - SELF-DIAGNOSTICS)DIAGNOSTIC TESTSDTC P0420: CATALYST SYSTEM LOW EFFICIENCYDIAGNOSTIC PROCEDURES
2008 Chevrolet Chevy Express G2500, Van Cargo, 6.6 6

Diagnostic Procedures

2008 Chevrolet Chevy Express G2500, Van Cargo, 6.6 6SECTION Diagnostic Procedures
WARNING: This page is about a different car, the 2003 Pontiac Vibe. 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, perform diagnostic system check - engine controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM.
  2. Check any for any HO2S DTCs and diagnose those DTCs first. Are DTCs P0130, P0133, P0135, P0136, or P0141 set? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, go to next step.
  3. Install scan tool. Start engine. Run engine until normal operating temperature is reached. Verify the engine is operating in closed loop. Run engine at 2500-3000 RPM for 3 minutes. Monitor HO2S 1 and HO2S 2 signals on scan tool. Is HO2S 2 signal as active as HO2S 1 signal? If yes, go to step  5. If no, go to next step.
  4. Use scan tool to clear DTCs. Turn ignition switch to OFF position for 30 seconds. Start engine. Operate vehicle within CONDITIONS FOR RUNNING DTC  or until catalyst test is complete. Did a DTC P0420 set? If yes, go to next step. If no, go to DIAGNOSTIC AIDS .
  5. Visually and physically check the 3-way catalytic converter for the following concerns:
    • Any dents.
    • A severe discoloration caused by excessive temperatures.
    • Any holes.
    • An internal rattle caused by a damaged catalyst.
    Ensure the 3-way catalytic converter is a high quality part and meets OEM specifications. Was a problem found? If yes, go to step  8. If no, go to next step.
  6. Visually and physically check exhaust system for leaks, damaged, loose or missing hardware. Repair as necessary. Was a repair necessary? If yes, go to step  9. If no, go to next step.
  7. Visually and physically check HO2S 2 for the following conditions:
    • The pigtail and the wiring harness making contact with the exhaust pipe.
    • A loose HO2S 2.
    • Any road damage.
    Repair as necessary. Was a repair necessary? If yes, go to step  9. If no, go to next step.
    8. NOTE: Locate and repair cause of catalytic converter failure before installing replacement converter.
  8. Replace 3-way catalytic converter. After repair, go to step  10.
  9. Start engine. Run engine until normal operating temperature is reached. Verify engine is operating in closed loop. Run engine at 2500-3000 RPM for 3 minutes. Monitor HO2S 1 and HO2S 2 signals on scan tool. Is HO2S 2 signal as active, as HO2S 1 signal? If yes, go to DIAGNOSTIC AIDS . If no, go to next step.
  10. Use scan tool to clear DTCs. Turn ignition switch to OFF position for 30 seconds. Start engine. Operate vehicle within CONDITIONS FOR RUNNING DTC  or until catalyst test is complete. Does DTC run and pass? If yes, go to next step. If no, go to step  2.
  11. Using scan tool, observe stored information, CAPTURE INFO. Does scan tool display any DTCs that have not been diagnosed? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, system is okay at this time.
RENDER: 1.0x

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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.