Fill AC Refrigrant (FREON) - AC System malfunctions and even completely shuts down if the Refrigerant pressure is low or if it is high. If the pressure is high ,the ac may shut down usually in the summer days, when the outside temperature is quite high. So when the AC starts blowing warm air, you know that it is time to check the freon pressure . Ofcourse the assumption here is that the all else is fine :- AC Belt , The AC compressor itself , and the hoses and pipes have now leak and sensors are ok.