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DTC P0420: Catalyst System Efficiency Below Threshold: Condition

DTC is set when value from HO2S-2 becomes closer to value from HO2S-1. MIL is illuminated when ECM detects malfunction during 2 consecutive drive cycles. Possible causes are:

Catalytic converter malfunction.
Exhaust system malfunction.
Fuel system malfunction.
HO2S-1 malfunction.
HO2S-2 malfunction.