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DTC P0420: Catalyst 1/1 Efficiency Failure: Testing

NOTE: If a HO2S DTC is also set, diagnose HO2S DTC before proceeding. Also, check for contaminates that may have damaged HO2S and catalytic converter such as contaminated fuel, unapproved silicone, oil and coolant.
  1. Turn ignition on. Using scan tool, read DTCs. If GOOD TRIP is displayed and equal to zero, go to next step. If GOOD TRIP is not displayed and equal to zero, go to step  7 .
  2. Inspect catalytic converter for dents, holes, leaks, internal breakage or severe discoloration caused by overheating catalytic converter. If any problem is found, replace catalytic converter. If no problems are found, go to next step.
  3. Start engine. Check for exhaust leak between engine and rear HO2S. Repair any leaks as necessary. If no leaks are found, go to next step.
  4. Start engine and let it idle. Check exhaust for excessive smoke caused by an internal engine problem. If no engine mechanical problems are present, go to next step. If any engine mechanical problems are present, repair as necessary.
  5. Review repair history. If downstream HO2S has been replaced without replacing upstream HO2S, replace upstream HO2S. If downstream HO2S has not been replaced recently or downstream HO2S has been replaced along with upstream HO2S, go to next step.
  6. If no other possible causes are remaining, replace catalytic converter.
  7. Conditions that set DTC are not present at this time. Perform the following to help identify intermittent condition:
    • Check for contaminates that may have damaged HO2S and catalytic converter such as contaminated fuel, unapproved silicone, oil and coolant.
    • Review repair history. If downstream HO2S has been replaced without replacing upstream HO2S, replace upstream HO2S.
    • With engine running at normal operating temperature, wiggle related wiring harness while monitoring related parameters on scan tool. If voltage changes or DTC sets while wiggling wiring harness, repair connectors or wiring harness where wiggling caused voltage to change or DTC to set.
    • Review freeze frame information on scan tool. Duplicate conditions present when DTC was set.
    • Refer to any Technical Service Bulletins (TSBs) that may apply.
    • Visually inspect related wiring harness. Check for chafed, pierced, pinched or partially broken wires.
    • Visually inspect related wiring harness connectors. Check for broken, bent, pushed out or corroded terminals.

    If a problem is found, repair as necessary. If no problems are found, test is complete.

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