A/C systems blow warm air for a variety of reasons. A few of these could be low charge on freon, vent door operations not working properly, and/or a compressor failure.
Efficient A/C systems in a vehicle will usually produce air that is 30 – 40 degrees cooler than outside temperature. When diagnosing an a/c system one of the top priorities would be to check the level of freon in the system. Visual inspections will confirm needed components such as belts and hose,that they are in good operating condition. In order to properly measure the amount of freon in a system special tools and equipment are required. It will need to be evacuated, measured, and weighed. A qualified licensed a/c technician will then determine the course of action required to repair the system.