The oxygen sensor, also known as a lambda sensor is an instrument for controlling the exhaust emissions of petrol, diesel and gas engines. It is an oxygen concentration sensor which measures the residual oxygen content of the exhaust gas and then transmits a signal to the engine control unit in the form of an electric voltage. The oxygen sensor voltage allows the control unit (ECU) to detect whether the mixture is too lean or rich. If the mixture is too rich, the control unit reduces the quantity of fuel in the A/F ratio and increases it if the mixture is too lean.