2000 e430 air conditioner blowing hot air
 

I have owned a 2000 e430 for 2 years, I keep having problems with the air conditioner blowing hot air. I have taken it in to different dealers as I travel. It seems they never have found a problem and it works when I get it backbut end up wit 200 to 300 bill. Could this just be a problem with pushing the wrong buttons on the panel ? Help would be appreciated:)