I'm guessing by A/C fan you mean the aux fan. It would help if you included some more information on the vehicle you are working on and maybe the reason you replaced the fan and what kind of fan you replaced it with etc. Maybe try checking/clearing any codes like mentioned in this thread.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)