The oxygen sensor feeds back the concentration information of the mixed gas to ECU by detecting the oxygen content in the engine exhaust gas, and it is installed on the exhaust pipe before the three-way catalyst.

The sensitive element of oxygen sensor used to generate voltage signal is zirconium dioxide (ZrO2), which has a layer of platinum on its outer surface, and a layer of ceramics on the outside of the platinum to protect the platinum electrode. The inner side of the sensing element of the oxygen sensor is exposed to the atmosphere, and the outer side passes through the exhaust gas discharged by the engine. When the temperature of the sensor is above 300℃, if the oxygen content on both sides is quite different, an electromotive force will be generated on both sides. The oxygen content in the inside of the sensor is high because it is ventilated to the atmosphere. When the mixture is thin, the oxygen content in the exhaust gas is high. The difference of oxygen content between the two sides of the sensor is very small, so the electromotive force generated by it is also very small (about 0.1V). However, when the mixture is too rich, the oxygen content in the exhaust gas is very small, the oxygen concentration difference between the two sides of the sensitive element is large, and the generated electromotive force is also large (about 0.8V). The heater inside the oxygen sensor is used to heat the sensitive element so that it can work normally.

If the oxygen sensor has no signal output or the output signal is abnormal, it will increase the fuel consumption and exhaust pollution of the engine, resulting in unstable idle speed, misfire and chattering. Common faults of oxygen sensor are:

1) Manganese poisoning. Although leaded gasoline is no longer used, the antiknock agent in gasoline contains manganese, and manganese ions or manganate ions after combustion will lead to the surface of the oxygen sensor, so that it cannot produce normal signals.

2) Carbon deposition. After the surface of the platinum sheet of the oxygen sensor is carbon-deposited, normal voltage signals cannot be generated.

3) There is no signal voltage output due to poor contact or open circuit in the internal circuit of the oxygen sensor.

4) The ceramic element of the oxygen sensor is damaged and cannot generate normal voltage signal.

5) The resistance wire of the oxygen sensor heater is burnt out or its circuit is broken, which makes the oxygen sensor unable to reach the normal working temperature quickly.