Each B Series and latest D and ECM Series are equipped with a pilot or diagnostic light. To determine where the problem is, turn the thermostat to its highest temperature setting. Go to the furnace, and if the pilot/diagnostic light is not on, the problem is within the thermostat or in its wiring connection to the furnace. If the pilot/diagnostic light is on, the problem is in the furnace. For earlier models of D and ECM Series, contact an electrician.