Furnace will not go on. It receives the signal from the thermostat but doesn't relay the signal. The light flashes once (which means check furnace). (The Fan works when you turn it from auto to on) Now what? Could it be the circuit board? How do you check? Merry Christmas! Help!

I would check the start/Stop settings on the furnace, may be the start setting is set too low. Or it could be that the thermostat is mounted close to a light bulb, so the thermostat is transmitting a high temperature reading to the controller.

What happens if you jump r to w at the furnace? If heat sequence starts, then you either need a new stat or stat wire.