System Description/Monitoring Procedure

Exhaust oxygen concentration at the upstream and the downstream of WU-TWC is detected from A/F sensor and HO2S respectively and accordingly ECM controls the closed loop which then controls the fuel injection volume. While the above controls is going on and if WU-TWC is in good condition, the output voltage of HO2S 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 at the exhaust oxygen concentration similar to that of the upstream of WU-TWC without being oxygenated or converted. Thus, waveforms of A/F sensor and HO2S output voltage become alike, ECM judges deterioration of WU-TWC by comparing waveforms of A/F sensor and HO2S.