[header] How to put freon in an ac unit [title] Check for vents blowing warm or room temperature air. [step] This is a tell-tale sign that your unit needs a refill of refrigerant. However, a broken thermostat may cause the same problem. 
If it's a refrigerant issue, the air temperature will steadily grow warmer over time because your unit is slowly losing refrigerant. [substeps] If a broken thermostat or other issue is causing the warm air, the temperature of the air will probably spike or fluctuate.