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) (see DTC TROUBLESHOOTING )

P0141: Secondary HO2S (Sensor 2) Heater (see DTC TROUBLESHOOTING )

Reset the ECM/PCM (see HOW TO RESET THE ECM/PCM ).
Start the engine. Hold the engine at 3,000 RPM with no load (in Park or neutral) until the radiator fan comes on.
Except F23A4 SULEV engine: Test-drive at 40-55 mph (64-88 km/h) for approx. 2 minutes. Then decelerate for at least 3 seconds with the throttle completely closed. Then reduce the vehicle speed to 35 mph (56 km/h), and try to hold it until the scan tool indicates FAIL or PASS as the TWC test result.
F23A4 SULEV engine: Test-drive at 50-55 mph (80-88 km/h) for approx. 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?
1. YES -Check the three way catalytic converter (TWC). If necessary, replace the TWC.
2. NO -Intermittent failure, system is OK at this time.