DTC P0420: Bank 1 Catalyst System - Low Efficiency: Description

Exhaust oxygen concentration levels are detected by oxygen sensors located before and after Warm-Up Three Way Catalyst (WU-TWC). Based on exhaust oxygen concentration levels ECM control closed loop operation of fuel injection. When TWC is functioning properly, variation cycle of HO2S No. 2 output voltage (oxygen concentration) is slower than that of HO2S No. 1 output voltage due to amount of oxygen in exhaust gas which has been stored in TWC. As WU-TWC deteriorates waveforms of HO2S No. 1 and No. 2 become similar. ECM judges deterioration of WU-TWC by comparing HO2S No. 1 and No. 2 output signal.