Interior fan comes on when the engine is off

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This is likely OK, especially if this occurs after driving with the A/C on, or the Defroster (which uses A/C). This will happen normally after driving with the A/C on to help remove moisture from the ventilation system and thus reduce "moldy" smells that occur when that moisture is allowed to sit in there. I would guess that you can verify this by looking in your owners manual...I found this in an online version: Passenger compartment blower The blower may come on automatically (certain models) after approximately 50 minutes, and stay on for 5 minutes, to remove excess condensation in the A/C evaporator.