if the furnace fails to come on it may be a bad thermostat
if the thermostat is activated and you do not hear the exhaust fan for the furnace come on it may be a bad thermostat
you can either place a jumper at the furnace where the two thermostat wires are located or if you only have a furnace thermostat without a/c then you can connect those two wires at the thermostat to bypass the thermostat
if the furnace operates it most like is a bad thermostat
turn off the main switch to the furnace prior
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