DTC P0420: Catalyst System Efficiency Below Threshold: Description

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