Rear Heated Oxygen Sensor (H02S-2)

The heated rear oxygen sensor, mounted behind the catalytic converter, monitors oxygen content in the exhaust to determine the efficiency of the catalytic converter. The H02S-2 is a four wire sensor, two which heat the sensor (B+ and ground), the other two which are the signal reference and ground. The sensor acts as an electro chemical cell, or battery to produce a voltage in response to the oxygen content in the exhaust. A bias voltage (voltage sent from the Powertrain Control Module (PCM)) is between 350 mv and 550 mv which is the voltage the PCM will see when the sensor is inactive or cold. As the heater and the converter temperature heat the sensor up to around 275° C (527° F), the sensor will produce its own voltage to counteract the bias voltage. Low voltage from the sensor indicates high oxygen content (lean mixture) whereas high voltage indicates low oxygen content (rich mixture). The wires to the sensor carry air reference which is used to compare differences in oxygen from the atmosphere to the exhaust stream.
The rear H02S-2 will react slower than the front OS2 due to the oxygen storage in the catalytic converter. The rear oxygen sensor is used only for converter efficiency, not for A/F ratio control.
The rear oxygen sensor operates between 275° C (527° F) to 850° C (1562° F).