When The ignition is off a fan is still running 4 about 2 minutes..why?

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Just got major engine work done ..a new intake manifold . now fan runs "on "after the engine turned " off" I see the fan is near the front end near the lever which opens the hood. is it related to a/c .?.i was told the a/c compressor needs replacing and a new thermostat. helpful info please .... the mechanic was not helpful in this regard. i never heard or saw this fan running before now. thank you