Heated Oxygen Sensor (HO2S)

HO2S detects oxygen content of exhaust gases and produces a voltage signal from zero to one volt. PCM uses this signal to determine system output signals which control air/fuel mixture.

Variations in voltage signal from HO2S serve as air/fuel ratio indicators. When oxygen content is low (rich mixture), voltage signal will be approximately one volt. When oxygen content is high (lean mixture), voltage signal will be approximately zero volts. By converting oxygen content to an electrical voltage, HO2S acts as a rich/lean switch.

HO2S contains a ceramic heater in the sensor housing. The heater operates on 12 volts. The heater is used in cold starts to help HO2S heat up quicker and to maintain the HO2S's Zirconia semiconductor at operating temperature.

In "closed loop" operation, PCM monitors HO2S input (along with other sensors) and adjusts the injector pulse width accordingly. During "open loop" operation (cold oxygen sensor), PCM ignores HO2S input and adjusts injector pulse width to a pre-programmed value based on other sensor inputs.