You can't access the stat wires from outside. From inside there should be four wires that connect to the furnace. Two of these wires will be the 12 volt power supply the other two will be the control wires that connect to the thermostat. I would disconnect the control wires at the thermostat if the furnace turns off after the cool down cycle I would replace the thermostat. If the furnace continues to run I would disconnect the control wires at the furnace. If this turns the furnace off I would look for a short in the control wires. If the furnace continues to heat after the control wares are disconnected I would check the control wires for a short inside the furnace. If no short is found replace the furnace control board.
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