I had this issue with my previous X5 and it was low on freon. I checked mine using my a/c gauges to verify and sure it was low. I had the driver side only blowing cold air while the passenger side was blowing just regular/not cool air. I added almost a bottle of R134a. But make sure you don't overfill system with freon since it will make your a/c not to blow cold air. I might have had a slow leak since a/c system should not lose freon.

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X5Diesel