The lambda sensor is mounted on the exhaust pipe in such way that one side of the sensor is in contact with the exhaust gases and the other with the outside air. When a lean mixture is burnt, an amount of oxygen is left in the exhaust gases. This will create a small difference in oxygen levels compared with the outside air and the sensor produces a low signal voltage of approximately 0 to 0.1 V. When a rich mixture is burnt, the exhaust gases contain no oxygen and this will create a high concentration difference compared to the outside air which results in a voltage of approximately 0.8 V. The ideal mixture is when the signal voltage goes from 0.1 V to 0.8 V or back. At that moment the combustion is optimal and the exhaust gases contain a minimum amount of carbon monoxide. The lambda sensor is only able to detect if a mixture is too lean or rich.
With the lambda sensor or zirconia oxygen sensor the concentration difference in oxygen levels of two gases can be measured. The sensor consists of a plate zirconium dioxide (zirconia) to which a thin layer of platinum is applied at both sides. When an oxygen concentration difference exists between the two sides, a voltage difference will be present between the two platinum plates. The value of the voltage difference is in relation with the concentration difference. The sensor needs to be at an operating temperature of 300°C to perform well and therefore the sensor is usually equipped with an electronic heating element.
The Lambda Sensor is at the heart of emission control continually monitoring the exhaust gas. Carbon Monoxide (CO) is produced by the engine. If the mixture supplied is too rich then CO will be high and visa versa. Having plenty of fuel (Rich) the engine will try to burn as much as possible, using up all available Oxygen, conversely if there is not sufficient fuel to maintain a correct burn then the excess Oxygen will pass into the exhaust system.