Rich trove of threads on diesel CELs and what to do...
Universally, the first step?:
1. Bring it to the dealer, get the CODES read-
Next? POST them here. Not "what the service sdvisor told you was wrong" but the actual BMW code. (4 digits, alpha and numeric)
There are hundreds of things that can cause a CEL to come on- it is way foolish to speculate...
In this thread the OP got bad advice from the "service engineer" (OMG, who could that be??? Oh, you mean the twit service advisor?)..your light comes on, EVEN if it goes off you want the code read and documented.
Keep it as simple as possible...but no simpler.