I use a sniffer in conjunction with UV dye. I just replaced the evaporator in my Miata this past week due to a pinhole leak. Sniffer picked something up, but UV dye confirmed it. After replacing the evap, I found a leak (using UV) at the high-side o-ring on the compressor. It evidently blew out when the TXV stuck closed (long story). Anyway, after replacing TXV, drier, evap, and some O-rings, I finally have A/C back in the car. I'm glad I chose the winter to work on it!

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280