5.0 Coyote is correct in that, someone not experienced in AC, can overcharge a system that does not need it, in addition, all AC systems have oil in them, most automotive ones, the oil mixes with the freon when the system is running, so sometimes when a system requires freon due to a leak it will have lost some oil, and may also require the correct type oil, and not too much, and it has to be added slowly. In addition, when adding freon, your cooling system actually gets worse, due to the freon from the can, and not circulating as much through the system, this is why, when a system needs some freon, I add some from the can, then shut the valve on the can, and let the system circulate to stabilize. This is not like adding oil to a car engine. The problem has to be properly diagnosed, then the appropriate action taken. I'm not saying you can't add freon if it needs it, but you have to be aware of the what needs to be done and the risks. Damaging a compressor could easily cost $600 to $1,00 parts and labor, because when these compressors grind or lock up, they many times flood the system with tiny metal particles, which has to be flushed from the system before a new compressor installed, system evacuated, and freoned back up. Good Luck.