Check engine on and off

DTC P0420: Catalyst 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/PCM.

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 at 50-55 mph (80-88 km/h) for about 2 minutes. Then decelerate for at least 4 seconds with the throttle completely closed. Then accelerate to 55 mph (88 km/h), and try to hold it until the scan tool indicates FAIL or PASS as the TWC test result.

Is the test result FAIL?

YES - Check the three way catalytic converter (TWC). If necessary, replace the TWC.

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