As the name suggests, a NOx sensor is used to measure the concentration of nitrogen oxides (NOx) in the exhaust gas of a diesel engine. The sensor consists of a zirconia element that is exposed to the exhaust gas. When NOx reacts with the zirconia element, it produces an electrical signal that is proportional to the concentration of NOx in the exhaust gas.
The output from the sensor is used by the engine control unit (ECU) to adjust the air-fuel ratio and timing of the injectors so as to reduce NOx emissions.