Huh?

I know you think this is the cars fault, the fault of the DEF system....and NOT a problem with 'your guy' or the quality of the work they are doing.

Seems that your mechanic believes it is/was one of the heater elements, and guessed at one.

Ive explained how one can definitively check each element...rare to find a tech that does that- they have no problem replacing $2500 worth of parts and - in the process- convince you that this is ALL bmws fault. The parts they replaced that were ok, all of it.