An oxygen sensor is an electronic device that measures the proportion of oxygen (O2) in the gas or liquid being analysed.The most common application is to measure the exhaust-gas concentration of oxygen for internal combustion engines in automobiles and other vehicles.
Automotive oxygen sensors, colloquially known as O2 (“ō two”) sensors, make modern electronic fuel injection and emission control possible. They help determine, in real time, whether the air–fuel ratio of a combustion engine is rich or lean.Watch the animated video showing how an oxygen sensor in the exhaust system of a car works.
Since oxygen sensors are located in the exhaust stream, they do not directly measure the air or the fuel entering the engine, but when information from oxygen sensors is coupled with information from other sources, it can be used to indirectly determine the air–fuel ratio.
The sensor does not actually measure oxygen concentration, but rather the difference between the amount of oxygen in the exhaust gas and the amount of oxygen in air. Rich mixture causes an oxygen demand. This demand causes a voltage to build up, due to transportation of oxygen ions through the sensor layer. Lean mixture causes low voltage, since there is an oxygen excess.via/read more: wikipedia