Comment by toomuchtodo

If it’s the high limit temp switch, that can be triggered by extended furnace runtimes when the temperature is so cold outside that the furnace must run nonstop to keep up with thermal losses (polar vortex and whatnot). This will potentially manifest in smart thermostats as frequent reboots, as the limit switch cuts off power to the furnace (which cuts power to the thermostat) to prevent it from bursting into flames. I can’t speak to Nest thermostats, but ecobee will send you an alert.