BMW Questions? Ask a BMW Repair Expert for Answers.
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Most likely cause is a Green/Brown wire broken under the insulation making a good connection intermittently - could mislead you into thinking it's good when the DME flags a code after you disconnect it. A more accurate way would be to leave it connected then wiggle it and see if it then flags the code. To confirm this, you could run a second wire, in fact creating a redundant circuit. You'll know you've found the cause if the issue goes away.
Please let me know if that helps.
Which alternator, Bosch or Valeo?
Oops, forgot to ask the most important question; what are you using to communicate with the DME, ISTA-d or O.S.S.?
I'm afraid all I've got on BMW procedures for this kind of issue is based on data obtained through either ISTAd or OSS (see attached graphic). I just can't see how that's going to help you if you don't have access to either software. Would you like me to opt out and re-open the question to the field to see if another expert has a way of working with the tool you do have? - optionally you could just ask customer service for a refund.