Why does my thermostat not work? Why does my thermostat readout -AC?

 

The thermostat communicates both with the indoor furnace and the outdoor unit. If you don't know what you're doing, changing it out yourself could cause additional problems. We have so many customers that, when having problems with their A/C unit, will immediately change out the thermostat when, 9 times out of 10 the problem is elsewhere. So you may want to save yourself the time and the headache, bite the bullet, and call an air conditioning company to properly diagnose your system. However, if your thermostat reading says -AC this is usually an easy fix. It can also apply if your system suddenly stops working inside and outside (especially if someone has been in the attic recently). -AC means that the indoor furnace has lost high-voltage power. There is a switch in the attic, usually near the attic pull-down or near the furnace. It looks just like a light on-off switch (but it's NOT the light, that's a separate switch) and I can't tell you how many times we go to homes and the only problem is that this switch has been turned off. So if you get the -AC reading on your thermostat or if your system suddenly stops working make this your first check.