Happen to me too buddy. Paid $5k for reco transmission unit. Drove it around for a few weeks and it came back on and now BMW is blaming it on my aftermarket mod. Wish they had checked that first, i told em that and they say its a different fault code heh.
Anyways they said to fix it again will be 3.5k for a transmission control unit. **** me...dunno what to do.