Like the air conditioner in your home, your car's air conditioning system blows cool air so you can feel cool throughout the hot summer season. But unlike with your residential AC system, you have the privilege of enjoying the cooling service provided by your car AC unit wherever you go by car. Car AC systems occasionally need to be re-gassed to restore refrigerant levels — refrigerant is the vital working fluid that helps cool the car interiors by drawing heat from the hot air entering the system via the return air grilles. Occasional re-gassing is normally required because refrigerant tends to leak from the system little by little until there isn't enough working fluid to ensure reliable cooling service.  

If it is not yet time for periodic re-gassing yet your car AC system is exhibiting the following symptoms, you may be low on refrigerant and need re-gassing. 

Warm air coming out of the air supply vents.

Your car's AC system supplies cool air to your car interiors via the air supply vents within your car. If your AC temperature settings seem okay yet your AC system is blowing out warm air, chances are high that the system could be low on refrigerant. Hotter-than-usual temperatures when your car AC is turned on is a tell-tale sign you may need a re-gas.

Prolonged hissing sound generated by your AC system.

The AC systems installed on most modern cars are designed to run as quietly as possible, but some sounds can still be heard even when the AC is operating at peak efficiency. One of those characteristic sounds may be a short hissing sound indicating that refrigerant is expanding within the system. However, if the hissing sound does not seem to die down after a while, then that could be the sound of refrigerant escaping from the system. To remedy the issue, you will need to have the leaks sealed and add more fluid to the system.

Visible signs of refrigerant loss.

If you can hear a hissing sound but also see refrigerant leaks, you should know that the refrigerant will continue to escape until the entire system stops working completely. If you see any signs of dark residue spots on any of the AC components, then this could be a visual clue that you have refrigerant leaks. 

If you detect any of the above signs, call in a professional car AC technician and ask them to check your AC system.