Abstract: A carbon-monoxide detector and alarm circuit for alerting occupants of a structure to a presence of a toxic gas. The apparatus utilizes a probe to detect carbon-monoxide within an air duct of a conventional furnace installation. A siren and a flashing red light are both activated upon a detection of a predetermined amount of carbon-monoxide within the air duct, thereby alerting the occupants to the presence of such carbon-monoxide. The apparatus also includes a warning sign which deploys upon activation of the alarm circuit.