Getting your dealer to pay for a/c problems at no charge.

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Hi: My wife drives a 2008 328xi. I know this is the X3 section, but it's where I found the most info on all the a/c-related problems in the 3 Series. Anyway, our car has been in for two a/c problems in the last year, which were covered by the four-year service warranty. The a/c stopped blowing cold air a week or so ago, and I took it to a local garage that found the freon was very low. They charged it and did a dye test, which revealed nothing. The service guy at the shop mentioned how many BMW's are having a/c problems, frequently in the evaporator core. He recommended talking to the dealer to avoid having to pay the garage for a hefty repair. When I called the dealer service department, they were very reluctant to look at the car, telling me to keep driving it to see if charging the system worked. I can already tell the air is not blowing as cold as it should. I want them to inspect it and repair it. Since it looks like a system problem, I don't want to be on the hook for it. Any advice? Thanks!