The heater/AC fan on my 1997 528I BMW goes on and off at various times. Why might this be happening.

Question

It only happens for a second or at the most 30 seconds at a time.

Answer 1

It is your Final Stage Unit (FSU) controller. There was a design flaw on these for this car. It has been redesigned and is available at you local dealer. If you are handy with cars and can stand on your head for a few minutes, it is fairly easy to replace. On the fron passenger side of the car, remove the kickpanel that is next to the drivetrain hump. Then look up under the dash and yoiu should be able to see the old FSU. There is a small hard plastic tab that hold it in its socket. Press hard on this tab and the FSU can then be removed. Then just push the new one in place.

The part should cost around $90 and it should not cost more than $60 to have a mechanic replace it.