NOx refers to nitrogen oxides.
NOx control is a major part of modern emission management, particularly on diesel vehicles.
NOx control can involve several systems working together:
EGR,
SCR / AdBlue,
NOx sensors,
ECU calculations.
If a vehicle reports a NOx-related fault, the sensor itself is not necessarily the only possible cause.
Wiring, dosing systems, exhaust components and ECU calculations can also be involved.