Circuit Description

Exhaust oxygen concentration at the pre-catalyst and the post-catalyst of WU-TWC is detected from HO2S-1 and HO2S-2 respectively and accordingly ECM controls the closed loop which then controls the fuel injection volume. While the above control is going on and if WU-TWC is in good condition, the output voltage of HO2S-2 is maintained at specified level. As WU-TWC becomes deteriorated, even when the above control is going on, the exhaust gas which has passed WU-TWC then passes HO2S-2 at the exhaust oxygen concentration similar to that of the pre-catalyst without being oxygenated or converted. Thus, waveforms of HO2S-1 and HO2S-2 output voltages become alike. ECM judges deterioration of WU-TWC by comparing waveforms of HO2S-1 and HO2S-2.