NOx – a nitrogen oxide sensor is a high-temperature device/sensor built/designed to detect nitrogen oxides in combustion environments such as an automotive or truck exhaust emissions.
The development of exhaust gas NOx sensors started in the 1990s. The most recent area of NOx sensor application are SCR systems for light and heavy duty diesel engines. To satisfy various OBD requirements, SCR systems typically use a NOx sensor downstream of the SCR catalyst. If excessive NOx or ammonia concentrations exist at the SCR outlet, an OBD malfunction/fault will be triggered, as NOx sensors are sensitive to both gases. Should such fault/error be detected, vehicle ECU will start limp mode timer, and, after a certain number of hours, starts etc.. (different from one manufacturer to other) the vehicle will enter limp mode.