Oxygen Sensor: Description and Operation

Heated oxygen sensor (HO2S)






The oxygen sensors consist of a titanium metal sensor surrounded by a gas permeable ceramic coating. Oxygen in the exhaust gas diffuses through the ceramic coating on the sensor, and reacts with the titanium wire altering the resistance of the wire. From this resistance change the ECM calculates the amount of oxygen in the exhaust gas. The injected fuel quantity is then adjusted to achieve the correct air/fuel ratio, thus reducing the emissions of carbon monoxide (CO), hydrocarbons (HC), and oxides of nitrogen (NO2). Two HO2 sensors are fitted, one in each exhaust downpipe just ahead of the catalyst. Note that if the wiring to these sensors is crossed, the vehicle will start and idle correctly until the sensors reach operating temperature. Then the ECM will read the signals from them and send one bank of cylinders very rich and the other very weak. The engine will misfire, have a rough idle and emit black smoke, with possible catalyst damage.

In the event of sensor failure, the system will default to 'open loop'. Operation and fuelling will be calculated using signals from the remaining ECM inputs.

The fault is indicated by illumination of the malfunction indicator light (MIL). ECM diagnostics also uses HO2 sensors to detect catalyst damage, misfire and fuel system faults.

North American vehicles have two extra HO2 sensors mounted one after each catalyst. These are used to determine whether the catalysts are operating efficiently.


CAUTION: Although robust within the vehicle environment, HO2 sensors are easily damaged by dropping, excessive heat and contamination. Care must be exercised when working on the exhaust system not to damage the sensor housing or tip.