If you have an Air Conditioner blowing warm, there's a number of things it could be. It's typical for people to brush it off and say 'it just needs a recharge' - well, if that's the case you have to ask why it needs recharging. Unlike older AC systems which had porous hose and inefficient joints, the AC in an E39 uses barrier hose, R134 refrigerant and high quality connections and o-rings. In other words, they don't leak unless there is a problem. You can certainly try refilling the system with gas - if you get cold air afterwards, you know that low gas was the problem - but the length of time the cold air lasts for might be measured in days rather than months or years.