automatic transmission size

going to need more info but im gonna assume you have a 99-05 3 series since they are known for this exact issue. going to need to open up the trans or replace. you can try flushing and refilling the trans but thats a 50/50 shot.

IF your automatic transmission will go forward with no problem but will not go backwards and all your gear slections (PRND) are in sync then your problem is an internal transmission failure. The transmission should be rebuilt or replaced. The automatic transmissions often fail because the fluid is not changed. The service recommendation that the transmission fluid is a lifetime fluid in any car is directly related to the lifetime you want out of the transmission. Meaning the fluids lifetime is over when it has now ruined your transmission.

this is correct guys, at this point i don't think a flush will help. you'll have to repair your transmission