Heated Oxygen Sensor (HO2S) consists of zirconium and alumina and is installed both upstream and downstream of the Catalytic Converter. It varies in accordance with the air/fuel ratio.

The sensor must be hot in order to operate normally. To keep it hot, the sensor has a heater which is controlled by the ECM via a duty cycle signal. When the exhaust gas temperature is lower than the specified value, the heater warms the sensor tip.