How do you know if you have a leak in the central air unit?

The system will not cool if it is low on refrigerant. Check the suction line and see if it feels cold and is sweating. The suction line is the larger of the 2 and is covered in black insulation. If it is not sweating or if it is warm to the touch, you are probably low on refrigerant. If you have to have refrigerant added more than once, it's a sure bet there is a leak in the system.j3h.