DTC P0420: Catalyst System Efficiency Below Threshold: Condition

DTC is set when while vehicle is being driven at constant speed other than at a high load, value from HO2S No. 2 becomes closer to value from HO2S No. 1. ECM monitors this circuit once per drive cycle. MIL is illuminated when ECM detects malfunction during 2 consecutive drive cycles. Possible causes are:

• Exhaust system leak.
• Catalytic converter malfunction.
• Fuel system malfunction.
• HO2S No. 1 malfunction.
• HO2S No. 2 malfunction.