Nox sensor

Can nox sensors be cleaned as  mine is sending a very intermittent engine management warning message (on the dashboard).   No symptoms other than the warning light from time to time, with no regularity. Sometimes can go weeks without coming on, and then several times a trip on other times.        The other sympton is that very occasionally the fan stays on for about 5 mins after I have stopped, but again it is sporadic and no common theme to when this happens.  The  particulates filter is OK.   Fuel consumption very good between 55 to 60 mpg on runs and slightly less on short journeys. No smells of diesel inside the car, and no smoke out the back.    The recent test (contrôle technique in France) showed very low readings from the exhaust.   My concern is that the engine will go into limp mode when I'm out and about.    Husband has a diagnostic thingy which came up as nox sensor, but as it's really intermittant, sometimes it reads this, and sometimes it reads nothing wrong.     Sorry to post such a blond question, although I am brunette! 😆