A NOx sensor should be replaced if a check engine light indicates a fault, when diagnostic trouble codes related to emissions performance appear, or when signs of failure occur. Common symptoms include poor fuel efficiency, excessive DEF consumption, loss of power, or the SCR system failing to maintain compliance with emission testing.

With the introduction of Euro 6 and EPA 10/13/17 emission standards, accurate measurement of nitrogen oxides became essential for meeting regulatory limits. Bosch NOx sensors provide continuous feedback for Selective Catalytic Reduction (SCR) systems, ensuring correct DEF dosing, reducing harmful NOx emissions, and supporting onboard diagnostics. Without NOx sensors, vehicles can not reliably comply with modern emission regulations.

The functional principle behind the NOx sensor is based on a ceramic electrochemical (amperometric double chamber) design. Exhaust gas passes over the sensing element, where selective electrochemical reactions measure concentrations of nitrogen oxides, oxygen, and even ammonia. The integrated Sensor Control Unit (SCU) processes these signals and communicates real-time data via CAN-bus to the ECU, enabling precise SCR dosing and emission control.