Air blows hot after awhile. on 2009 Nissan Altima

When the AC is first turned on it blows very cold, then after a while it starts to blw hot air.

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2 answers
if you go out three days in a row and each day the a/c starts out really cold but gets slowly warmer ..say after about 20 minutes then your not low on refrigerant (unless theres a ghost re-filling it at night) The next time you have a chance , open the hood , find the a/c compressor ,make sure you can see the compressor clutch(belt goes around it)at the end.Thenturnthe key to 'on' not run and turn your a/c ,defrost controls to 'off'.Now have someone start the car while you carefully observe the a/c clutch. With a/c turned off , belt pulley should be spinning (yeah-duh) but the clutch should not be.Assuming it's not engaged now , go ahead and get your helper to turn on the a/c (on vent setting with fan on high and windows down)turn temp to coolest. Watch the fan clutch(caution may get boring for a minute or three.................THEN BAMM........okay not really , leave everything the same except turn the fan speed to low and continue watching the fan clutch and listening.By now ,if it hasn't already , it should have cycled on and off or it should start cycling soon two -three minutes max.IF compressor isn't cycling NOW you need to bring it to a trusted place for diagnosis of the control system. OR if it' hard to find a shop you trust the dealer can usually be the easiest to hold accountable for their diagnosis and repair even though they may be pricier initially. good luck
POSS FREON LEVEL ISSUE HAVE IT CK'D BY AN A/C MECH TO DETERMINE IF CHARGE IS NEEDED OR SOMETHING ELSE IS WRONG