An oxygen sensor (or lambda sensor) is an electronic device that measures the proportion of oxygen (O2) in the exhaust gas.
Oxygen sensor determines the air–fuel ratio of a combustion engine is rich or lean. 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.