DTC P0420: Three-Way Catalyst (TWC) System Efficiency Low

Inspect exhaust system for damage or leaks and repair as necessary. If no problems were found with exhaust system, check operation of Heated Oxygen Sensor (HO2S). See DTC P0136: HEATED OXYGEN SENSOR (HO2S) NO. 2 CIRCUIT MALFUNCTION  . Repair as necessary. If HO2S is okay, replace TWC and retest system.