The NOx sensor is part of the NOx reduction aftertreatment system used in diesel vehicles with urea based SCR systems. The sensor located upstream of the SCR catalyst directly measures the engine-out NOx gas concentration, which helps determine the optimum amount of urea injection. The NOx sensor downstream of the SCR catalyst is used to monitor the performance of the catalyst. Accurate NOx measurement is in high demand to comply with increasingly strict NOx emissions regulations. The NOx sensor is also capable of measuring O2 concentration, just like a wide range air-fuel sensor.