On my 1998 318i the petrol filler flap lock will no longer lock with the central locking. If I manually lock it from inside the boot it will unlock with the central locking but not lock again. I have had the lock out and it locks and unlocks okay on a 12V battery when I toggle the polarity to it. So I guess there must be a fault in the central locking module which must toggle the polarity of the voltage to the lock when I try to lock it. Probably not worth looking at for such a minor fault...unless someone knows different?