Well, my dealer removed and replaced the refrigerant free of charge. He said the level was correct. The car still makes the hissing noise, albeit the air con works fantastically. My wife says it has always made that noise. I am beginning to wonder whether it has always made the hissing noise, too. I suspect it has, and that I didn't pay any attention to it until a passenger asked what it was.

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JJ Rodger
2013 G350 Bluetec
1999 SL 500
1993 E300 diesel T
1990 190