First thing to go is the nox sensor, engine light comes on.
Into Mercedes for new sensor and software update, all fixed. (no goodwill from Mercedes, simply not interested)
Ten months later, the exact same thing happens again, all fixed, but we're now talking almost £2000 in repairs.
Less than a year later, engine light on again while on holiday, with intermittent message, "adblue level cannot be read" or similiar.
Before I got home from holiday, this message changed to "no start in 490 miles". I had my Icarsoft with me and managed to rest this message twice, on the 3rd occasion it refused to reset and continued the countdown.
Had Mercedes specialist look at it and was informed it needed new adblue tank, £1200 plus labour.
Local remappers could delete adblue for a few hundred quid, so over 3 years later, not had anymore adblue faults and car runs like a dream.