This quick video shows owners of Bryant natural gas furnaces the location of the flame sensor (one white wire). The main symptom of a dirty, cracked, broken or faulty flame sensor is repeated short cycling. The furnace will turn on and then turn off quickly after 5, 10, 15 or 30 seconds over and over again. If the flame sensor does not detect a flame, the control board will shut down the furnace to prevent from having unburnt natural gas from flowing into the heat exchanger.