Heated Oxygen Sensor Monitor

OBD-II regulations require monitoring of upstream Heated Oxygen Sensor (HO2S) to detect if deterioration of sensor has exceeded emission thresholds. An additional HO2S is located downstream of Warm-Up Three-Way Catalytic (WU-TWC) converter, or after pre-catalytic converter to determine efficiency of catalyst. Although downstream HO2S is similar to type used for fuel control, it functions differently. Downstream HO2S is monitored to determine if a voltage is generated. That voltage is compared to a calibrated acceptable range.