P0420 Catalyst Efficiency (Bank 1)

P0420 Catalyst Efficiency (Bank 1) - The Powertrain Control Module (PCM) detects a failed catalyst if the final downstream O2 sensor state of change test ratio is within the calibrated fail threshold. When monitored DTC P0420 Catalyst Efficiency (Bank 1) No other O2 sensor circuit faults active.
There are several different catalyst efficiency monitoring windows. Each specific window has several difference enable conditions. The best opportunity for the catalyst effeciency monitor to run is, engine temperature above 70°C (158°F), engine speed between approximately 1000 rpm and 3000 rpm, vehicle speed between 20 mph and 85 mph, under moderate engine load. The MIL light illuminate.

    Theory Of Operation O2 sensor


    Possible causes P0420 code

    • O2 sensor 1/1 return circuit shorted to ground.
    • O2 sensor 1/2 return circuit shorted to ground.
    • O2 sensor 1/1 return circuit open.
    • O2 sensor 1/2 return circuit open.
    • Exhaust leak.
    • Engine mechanical condition.
    • Aging O2 sensor.
    • Catalytic converter.

    Diagnose code P0420 Catalyst Efficiency (Bank 1)


    (Sorry!!! Step 1-6, refer to code P0430)
    Step 1. Check for an active DTC
    Step 2. Visually inspect catalytic Converter
    Step 3. Check the O2 sensor 1/1 return circuit voltage
    Step 4. Check the O2 sensor 1/2 return circuit voltage
    Step 5. Check the O2 sensor 1/1 return circuit for a short to ground
    Step 6. Check the O2 sensor 1/1 return circuit for an open/high resistance

    Step 7. Check the O2 sensor 1/2 return circuit for a short to ground
    Turn the ignition off, disconnect the O2 sensor 1/2 harness connector. Check for continuity between ground and the O2 sensor 1/2 return circuit in the O2 sensor 1/2 harness connector.

    Is the continuity between ground and the O2 sensor 1/2 return circuit?

    YES, repair the O2 sensor 1/2 return circuit for a short to ground.
    NO, go to step 8.

    Step 8. Check the O2 sensor 1/2 return circuit for an open/high resistance
    Measure the resistance of the O2 sensor 1/2 return circuit from the O2 sensor 1/2 harness connector to the PCM harness connector.

    Is the resistance below 3.0 Ohms?

    YES, go to step 9.
    NO, repair the O2 sensor 1/2 return circuit for an open or high resistance.


    Step 10. Check the exhaust system for leaks
    Perform the CHECKING THE EXHAUST SYSTEM test procedure.

    Were any exhaust leaks found?

    YES, repair or replace the leaking exhaust parts as necessary.

    NO, go to step 11.

    Step 11. Engine mechanical failure
    Check the exhaust for excessive smoke caused by an internal problem in the engine.

    Is an engine mechanical condition present?

    YES, repair the engine mechanical condition as necessary.
    NO, go to step 12.

    Step 12. Aging O2 sensor
    A new Downstream O2 sensor along with an aging Upstream O2 sensor may cause the DTC to set. Review the vehicles repair history.

    Has the Downstream O2 sensor been replaced without replacing the Upstream O2 sensor?

    YES, replace the front O2 sensor as necessary.
    NO, go to step 13.

    Step 13. Catalytic Converter

    If there are no possible cause remaining, view repair.

    REPAIR, replace the Catalytic Converter

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