On the other hand it may be just the heater.The current BMW diagnostic system ,ISTA,is accurate enough,plus there are quite good self-test capabilities built-in the control modules.Pure electric fault,open or short circuit,or resistance out of specs,are easy to determine.But having a paper trail is never a bad idea.
That DEF tank is problematic,especially in the early models,but the European tuner I am working with is almost ready with a solution for DPF and DEF off.No CEL,no emission stuff,never put more DEF or buy parts for the tanks.