Heated Oxygen Sensor (HO2S) consists of zirconium and alumina and is installed on upstream and downstream of the Warm-up Catalytic Converter (WCC).

After it compares oxygen consistency of the atmosphere with the exhaust gas, it transfers the corresponding voltage signal to the ECM. When A/F ratio is rich or lean, it generates approximately +1V or 0V respectively.

In order that this sensor normally operates, the temperature of the sensor tip must be higher than predetermined temperature. So it has a heater which is controlled by the ECM duty signal. When the exhaust gas temperature is lower than the specified value, the heater warms the sensor tip.