P0420

DTC P0420: Catalytic System Efficiency Below Threshold

NOTE: If some of the DTCs listed below are stored at the same time as DTC P0420, troubleshoot those DTCs first, then recheck for DTC P0420.

P0137, P0138: Secondary HO2S (Sensor 2)
P0141: Secondary HO2S (Sensor 2) Heater

1. Reset the ECM.
2. Start the engine. Hold the engine at 3,000 rpm with no load (in Park or neutral) until the radiator fan comes on.
3. Test-drive 40 - 55 mph (64 - 88 km/h) for approx. 2 minutes. Try to keep the engine speed between 1,500 to 2,500 rpm. Then decelerate for at least 3 seconds with the throttle completely closed. Then reduce the vehicle speed to 56 km/h (35 mph), and try to hold it until the scan tool indicates catalyst readiness complete.

Is the test result FAIL?

YES - Check for crack or melting in the three way catalytic converter (TWO). If it is damaged, replace the TWC.

NO - Intermittent failure, system is OK at this time.