OBD System Description - Catalyst Monitor

OBD System Description - Catalyst Monitor

System Description / Monitoring Procedure

Exhaust oxygen concentration at the upper part and the lower part of WU-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 controls 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 upper part of WU-TWC without being oxygenated or converted. Thus, waveforms of HO2S-1 and HO2S-2 output voltage become alike, ECM (PCM) judges deterioration of WU-TWC by comparing waveforms of HO2S-1 and HO2S-2.

Catalyst System Layout

The system has 2 oxygen sensors located upstream and downstream of the catalysts. The layout of catalysts and oxygen sensors is described below.

Catalyst and HO2S Layout





DTC Description / Detecting Condition / Confirmation Procedure

P0420

Refer to P0420 DTC P0420: Catalyst System Efficiency below Threshold.

Catalyst Monitor

Operation





Enable conditions





Typical malfunction thresholds






MODE $06 Data