DTC P0420: Catalyst System - Efficiency Below Threshold

NOTE: Trouble shoot DTC P0136, P0139 or P0141, if set, before proceeding.

If DTC P0420 is set, check Three-Way Catalytic Converter (TWC). Check for exhaust system leakage. Repair or replace components as necessary. If no problems are found, go to next step.
Perform PCM reset procedure. See PCM RESET PROCEDURE/CLEARING CODES under SELF-DIAGNOSTIC SYSTEM. Start engine and run at 3000 RPM with no load and transmission in Park or Neutral, until radiator fan comes on. Install SCS service check connector. See Figure.
Test drive vehicle at 40-55 MPH for at least 2 minutes. Decelerate with throttle completely closed for at least 3 seconds. Reduce speed to 35 MPH and try to hold until readiness code comes on. If DTC P0420 sets, go to next step. If DTC P0420 does not reset, problem is intermittent. System is okay at this time.
Remove TWC. Shine a flashlight into catalyst inlet and check for plugging, melting or cracking. Replace TWC and/or WU-TWC if necessary.