Within the past week, the weather has been radical, ranging from cold nights to warm days. I had this problem during the winter as well, and I just used a portable ceramic heater to heat my room when I slept. I was afraid of letting it run while I was away, weary of it overheating the home and causing a meltdown. The unit had cycled "on/off" in both modes (cold and hot) before as usual, but just recently, it would not cycle "Off" when it reached it's specific temperature. This occurred more during the "Heat" cycle. That would be the reason I left the power at the breakers set to "Off" while I was away. I tried to run the A/C yesterday, and it would blow warm air. The outside unit is running, and it would cycle "on/off" when I moved the stat. Even after flicking the switch from "Heat/Off/Cold" quickly to try to unstick what might seem to be stuck did not work this time. While the switch is set to "Off" and the fan set to "Auto", the unit will blow and blow hot air at that. When I do set it to "Cool", the outside unit runs, but you can hear the inside unit cycling the furnace. Therfore, heat overpowers cold. During the winter, the heater would not even heat, but would blow. After I gave up on it and left it off (I left the breakers off as well), I tried to run it again a week later. It started blowing hot again, but it would not stop when it reached it's set temp. Sometimes, I wouldn't even know if it was running right, because I would have it set on 72 degrees, and the digital gauge I have on the wall would show at least 77-78 degrees before it finally shut off. Other times, it would shut off at around 74 degrees (3-4 degrees makes a big difference when it feels like you're in a sauna). Sometimes, I would try to just turn it off, but it wouldn't. Even after a few minutes of it being set to "Off", it would just keep blowing. I would have to shut it down at the breakers. This is a very frustrating problem, and I would like to know what the problem could be without calling for a repair. Could this be caused by a simple fix? I did some searching, and looks like I might have a defective thermostat. Is this the root of this heating problem? Any ideas are welcomed, thanks.