Sorry just re-read your post. I'm not sure about the X5 but my 540 which is similar has two fuel pumps. One pumps the gas to the engine and the other pumps the fuel from one side of the tank to the other. There is a hump in the gas tank that goes over the drive line. It is the secondary fuel pump and if it fails it will not pump gas to the other side of the tank and will not throw a code. My fuel pump went bad at about 70,000 miles on my 540 so that might be the cause. It is easy to change by yourself and is located under the rear seat.