If you find that the cooling fluid (freon) has gone, it must be leaking problem. It could cost you lots of money to replace parts and labor cost is pretty high. I had the same problem before, and it costed me about $1,800 to repair it including parts and labor. BMW center told me that it was a part located behind the LCD display leaking cooling fluid, the leaking part must be replaced.

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--2004 E53 X5 4.8is - ECE version; right hand drive; Located in Hong Kong; ext. black sapphire metallic; int. leather black; adaptive dynamic bi-xenon headlights; 16:9 onboard monitor w/o navigation; 20" genuine BMW Y-spoke 214 wheels with Bridgestone Dueler H/P Sport tires; Int. rear view mirror w/ compass.