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2005 Yukon XL A/C Problems

I have a 2005 Yukon XL. In Oct 2010, my front A/C started blowing hot air even though it was set to cold. Nothing has happened since then, except that about a month ago, the A/C in the front stopped working but the A/C in the back still works great. The one in the front works when it wants to (once or twice in a week) and when it does, there is cold air coming out of it. We live in FL so it is getting pretty hot to not have A/C and we are going on vacation soon. Any ideas as to what can be causing my A/C to work like this?

There are a couple of possibilities.

First the blend door; its driving servo; or the front controler may not be allowing air to bypass the heater core. This is a HVAC control problem that would be independent of the auxiliary HVAC unit.

Second the orfice tube for the front unit may be restricted or the filter clogged. This is unlikely as you get cooling intermittantly. The auxiliary unit uses a TXV (expansion valve) which is a little more tolerant of debris than an orfice tube.

A good HVAC tech should be able to determine if the front evaporator core is cooling. By isolating the front unit from the auxiliary unit, the gauge set should be able to diagnose the front unit. Then, (s)he can determine if the blend door is responding to the commands of the temperature setting.

Hope this helps.