I have a Trane Gas Furnace installed in 2005, Model TUD120R9V5K 4 coupled with a Trane Programmable Thermostat. Been working fine without any problem. The heat is set at 68. Last night it came on as it automatically does, but would not turn off. Finally when it got up to 78, I turned the Thermostat to OFF and still would not turn off, kept blowing hot air. So I pulled the electric plug from the Gas Furnace and it turned off. Then an hour later I plugged it in again, and the Thermostat being off, did not start until I reset it to a higher heat. Then it started and automatically turned off. For now seems OK, but what would cause this? Defective relay? Thermostat problem or?? Thanks

It could be the thermostat or thermostat wiring but a sticking relay on the circuit board could also cause this problem.