Installed in both upstream and downstream of the Manifold Catalytic Converter, the Heated Oxygen Sensor (HO2S) consists of zirconium and alumina. The sensor output voltage varies in accordance with the air fuel ratio.

The sensor must be heated in order to operate properly. To keep it heated, it is equipped with a heater controlled by the ECM via a duty cycle signal. When the exhaust gas temperature is lower than the specified value, the heater will warm up the sensor tip.