We had a bad Aux Fan in our 2005 E53. Now I know, if I turn on the AC full blast and the Aux Fan doesn't come on with 30-60 seconds, something is wrong.

The funny thing is I remember when the X5 was new the Aux fan would run for a while after we turned off the engine. When it failed, I just didn't notice that any more. Mechanic told me it's not a well made part and is prone to failure.