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.

Reset the PCM (see HOW TO RESET THE PCM ), then do steps 2 through 5 to reset the readiness code.
Start the engine. Hold the engine at 3,000 rpm with no load (in Park or neutral) until the radiator fan comes on.
Drive for about 10 minutes without stopping. Your speed can vary.
With the transmission in D position, drive at a steady speed between 50-62 mph (80-100 km/h) for 30 seconds.
Repeat step 4 three times. Between each repetition, close the throttle completely for 1-2 seconds. If the engine is stopped during this part of the procedure, repeat steps 3, 4, and 5.
Check for a Temporary DTC with the scan tool or the HDS.
Does the scan tool or the HDS indicate Temporary DTC P0420?
1. YES -Check the TWC. If necessary, replace the TWC.
2. NO -Check for readiness code completion. If the readiness code is complete, it was an intermittent failure, and the system is OK at this time. If the readiness code is incomplete, repeat steps 2 through 5.