DTC P0420: Catalyst System Low Efficiency: Description

Engine Control Module (ECM) monitors oxygen concentration in the exhaust gas which has passed through Three-Way Catalytic Converter (TWC) by Heated Oxygen Sensor No. 2 (H02S-2). When TWC is functioning properly, variation cycle of H02S-2 output voltage (oxygen concentration) is slower than that of Heated Oxygen Sensor No. 1 (H02S-1) output voltage due to amount of oxygen in exhaust gas which has been stored in TWC.