MPG not registering correctly on my BMW 325i. Stuck and displays one number.

Hello, When these stop functioning correctly, they tend to come and go, so don't be surprised if it works again, flickers, or does something else silly. It is controlled by increasing or reducing power, so there may be damage to the circuit board inside the cluster that powers it. Alternatively, this may result from incorrect input, which would indicate an issue elsewhere. If other issues showed up as well, I advise a complete diagnostic systems check, just to be sure. However, if it is isolated to the MPG gauge, you can ignore it if you wish.