Cooling System Weirdness

[Eliot Sennett]

Last year my 2006 A6 quattro started blowing hot air out of the side 
vents off and on when the AC was on. Not all the time, but I can't find a
 pattern. I took it to the dealer who ran an analysis and said there is a
 valve somewhere in the system that is misbehaving and needs to be 
replaced, for $700. Does anyone know what this might be and whether I 
can replace it myself? Thanks.