DTC P0420: Catalyst System Efficiency Below Threshold

NOTE: If DTC P0137, P0138 and/or P0141 is set, troubleshoot those DTCs before proceeding.

Perform ECM/PCM reset procedure. See ECM/PCM RESET PROCEDURE/CLEARING DIAGNOSTIC TROUBLE CODES under SELF-DIAGNOSTIC SYSTEM.
Start engine and run at 3000 RPM, with no load (in park or neutral), until radiator fan comes on. Allow engine to idle.
Test drive vehicle on highway for about 10 minutes without stopping. Speed may vary.
With A/T in "D" position or M/T in 4th gear, drive vehicle at steady speed between 50 and 62 MPH for 30 seconds.
Repeat step 4 three times. Between each repetition, close throttle completely for 1-2 seconds. If engine is stopped during this procedure, go to step 3 and do procedure again.
Using scan tool, check for temporary DTC. If DTC P0420 resets, go to next step. If DTC P0420 does not reset, check for readiness code completion. If readiness is complete, problem is intermittent. System is functioning properly at this time. If readiness is incomplete, repeat steps 2 - 5 .
Inspect Three-Way Catalytic Converter (TWC) for plugging, melting, cracking, exhaust system leaks or TWC deterioration. Repair or replace as necessary.