No flashing or blinking lights or numbers. No read out at all on thermostat.

No batteries, it bleeds power off the furnace I believe. York natural gas furnace. I checked the breakers, they are all on. The pilot light is on.

This has happened before, about 2 weeks ago. I had a service guy come out, he tested stuff for hours, and found nothing definitive. In the process of tinkering, the thermostat came back on but the service tech could not tell us why. We left it at that. Now it is out again.

I know there is switch that shuts off the furnace to prevent overheating. Before I call for service again, I would like to try to reset that but I have no idea what it looks like or where it is located within/around the furnace.

It is a York. Model number P4UG012N06501A


Any info would be appreciated.

Go to the breaker and carefully throw it fully to the off position, and then back on. Sometimes they look on, but are off. If it was already on, it should also reset anything that needs reset. Pull the thermostat off the wall, and check all the connections. Wiggle the wires around. Pay careful attention to the Red and white ones. One of them could be broken off inside the insulation. Look for a suspiciously flexible place in the wire. Red is power from the transformer, and white energizes the heat cycle. Enough power to run the thermostat bleeds through it.

You might look inside under the cover for the filters. If you see a red button, it may be the reset.

I'd be looking for bad connections on the thermastat wires. At the furnace and at the termostat.