Abstract: A new and improved failure alarm apparatus is provided for a furnace and an air conditioner which selectively, alternately share a common thermostat. The furnace and air conditioner failure alarm apparatus includes a battery power supply for powering electrical circuits in case of AC power failure. A first sensor assembly circuit monitors thermostat status and is connected to a thermostat on a neutral wire (white wire) connected to the thermostat. Each of a second and third sensor assembly includes a respective temperature sensor and a respective temperature sensor circuit. A second temperature sensor is placed adjacent to an air duct outlet. In addition, a third temperature sensor (an environmental temperature sensor) is housed in the furnace and air conditioner failure assembly housing where the signal processing an alarm circuitry are housed.