Rear Ho2 Sensor

The rear (downstream) HO2 Sensor is located near the outlet end of the catalytic convertor. The rear HO2 Sensor input is used to detect catalytic converter deterioration. As the convertor deteriorates, the input from the downstream sensor begins to match the upstream sensor input except for a slight time delay.

By comparing the rear HO2 Sensor input to the input from the front sensor, the ECM calculates catalytic converter efficiency.