Oxygen Sensor: Description and Operation

CIRCUIT DESCRIPTION
Exhaust oxygen concentration at the pre-TWC and the post-TWC is detected from HO2S-1 and HO2S-2 respectively and accordingly ECM (PCM) controls the closed loop which then controls the fuel injection volume. While the above control is going on and if TWC is in good condition, the output voltage of HO2S-2 is maintained at specified level. As TWC becomes deteriorated, even when the above control is going on, the exhaust gas which has passed 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 (PCM) judges deterioration of TWC by comparing waveforms of HO2S-1 and HO2S-2.