The document discusses 4 common reasons for an Audi AC system blowing warm air: 1) Refrigerant leakage from old hoses or a punctured evaporator, which causes the antifreeze to evaporate without producing puddles. 2) A malfunctioning condenser that is clogged or unable to properly cool the refrigerant. 3) A damaged compressor that cannot circulate the refrigerant to be cooled. 4) Electrical system problems like damaged wires or blown fuses that prevent the system from functioning properly.