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

Check Heated Oxygen Sensors (HO2S). See DTC P0131  : HEATED OXYGEN SENSOR (HO2S) NO. 1 CIRCUIT VOLTAGE LOW, DTC P0132  : HEATED OXYGEN SENSOR (HO2S) NO. 1 CIRCUIT VOLTAGE HIGH and DTC P0136  : HEATED OXYGEN SENSOR (HO2S) NO. 2 CIRCUIT MALFUNCTION. Repair as necessary. If HO2S is okay, replace TWC and retest system.