P0421 MITSUBISHI Warm Up Catalyst Efficiency Below Threshold- Cylinders 1-4

Possible causes – Warm Up Three Way Catalytic converter deterioration or malfunction– Exhaust system leaks– Loose front or rear HO2Ss– Faulty front heater O2 sensor– O2 sensors harness is open or shorted– O2 sensors poor electrical connection Tech notes As

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