Oxygen Sensor: Description and Operation

The combustion gases, after passing through the exhaust manifold, enter the catalytic convertor, where the quality of the exhaust gas emission is modified. The quality of the exhaust gas emission is constantly checked by the Heated Oxygen Sensors (HO2S), which are situated both upstream and downstream of the catalysts. By comparing the signal outputs of pre and post catalyst sensors the ECM can make corrections to the fuel and ignition settings as necessary. The sensors contain integral Heaters which accelerate the warming-up of the sensors to enable a rapid correction of initial settings which may be causing the emission of low quality exhaust gases.






The NTK TO 19 HO2S is used in the three way catalytic converter system to accurately monitor exhaust gas for correct air/fuel mixture.

The sensors are of the Titanium Dioxide (TiO2) type, which have a tip, composed of an alumina substrate with a thick film titanium dioxide element. The TiO2 sensor does not require reference air to detect the oxygen content of the exhaust, so flooding or contamination of the sensor exterior will not affect sensor performance. The resistance of the sensor element varies greatly with the partial pressure of oxygen in the exhaust gas. This change in resistance is converted to a change in voltage output to the ECM via a constant voltage source and reference resistance. Whenever the air / fuel ratio passes lambda-1 (stoichiometric mixture), the sensor delivers a voltage swing, indicating whether the mixture is richer or leaner than lambda-1.

HO2S Characteristic:

Fig. 2 HO2S Circuit:

The HO2S resistance varies with temperature and so a heater is used to maintain a constant tip temperature of approximately 700 °C. At this temperature the sensor output will be 3.3 V for lean running and 1.1 V for rich running. It takes approximately 20 seconds from engine start for the sensors to reach an active temperature of approximately 500 °C.