I've got a Mini Cooper that has another 2k miles left on the warranty . My AC started blowing hot air over the weekend. I took it in for service this morning. They just called and said that it needed some refrigerant. They tested for leaks and could not find any. They used Dye. Am I right in thinking that an AC system is a closed system and shouldn't need refrigerant unless there is a leak? The warranty does not cover filling up refrigerant.