[TAKPAK]

We have a 2011 3400 as well. We have not had any problems with ours yet. Furnace turns on as set, and the AC comes on as set. But, you have to change the mode to whatever you want, AC, furnace, fan only, or "off". The thermostat itself (I'm assuming yours is on the wall next to the stairs) only has a "telephone jack" (modular) style plug in the back of it with signal conductors in a shield that runs to the AC unit in the ceiling. The AC unit is what actually has all the wires from the T'stat, furnace, and the AC itself on a terminal block behind a panel. It wouldn't surprise me if one of those conductors got cross connected. Otherwise, it is supposed to be impossible to do both simultaneously, because you have to "tell" the control which mode to be in. I'd check the terminal block on the AC (accessable from the inside), and it could possibly be a bad control module there as well.

Hope this helps.