What does the "check anti-pollution system" warning mean on a Renault Master?

On a Master 2.3 dCi Euro 6 (2019-2024), this warning, usually accompanied by fault code P2200 and, in more serious cases, a mileage countdown to a starting lockout, most often indicates a failed NOx sensor in the SCR (AdBlue) system. The engine has two of these sensors, one before and one after the catalytic converter, and each has a different part number. A diagnostic scan is therefore required before purchase to identify which sensor has failed. Do not allow the countdown to expire. Once it reaches zero, the vehicle will not start, even with a full AdBlue tank. After replacement, the sensor must be adapted using diagnostic equipment (CLIP), otherwise the fault will return. It is also worth checking the AdBlue injector and pump, which can also fail in this system.