'''++geef2++''' both will work but the freon detector is a lot neater, requires less preparation and will not require any change/adding of refrigerant. The UV light requires the dye to actually leak out under pressure somewhere. So, for that you would want to make sure that your engine and AC plumbing is relatively clean. You would also need to add a considerable amount of dye refrigerant to your AC system for it to show under the UV light.