The air-con in my car has stopped working, talking to some garages they do a vacuum test first, and if the system won't hold a vacuum the machine won't allow the system to be re-gassed. The vacuum test isn't perfect though, and can miss leaks. If the system is re-gassed with a dye added, this will show up where it's leaking, then you need to replace the leaking part AND pay to have it re-gassed again. On my old car it was found that the condenser was leaking, it was replaced under warranty but I still had to pay for the re-gassing.