If your car air conditioning vents blows out warm air even if your car engine is not under a lot of stress or within the operating temperature then that’s a different story and this means a problem. More often this kind of problem can be traced depending on the behavior of your car air conditioning system. If your car air conditioning vents initially blows out cold air then after quite a while it starts to blow out warm air then the problem can be traced to a freezing up expansion valve. If your air conditioning expansion valve freezes up the flow of refrigerant to the air conditioning lines will be block temporarily thus causing your car air conditioning vents to blow out cold air initially then afterward begins to blow out warm air. Your car expansion valve can only be freezing up if there is moisture on the refrigerant and this moisture can only be entering via leak on your car air conditioning system.