2 Pack Smoke and Carbon Monoxide Detector,Smoke Detector Carbon Monoxide Detector Combo with BatteriesTwo Sensors: Carbon monoxide detector smoke alarm has a 2 in 1 dual sensors, photoelectrical and electrochemical sensors. They work separately, no matter which one reaches their dangerous concentraion, combination smoke carbon monoxide alarm detector will go off with red led flashing Readable Digital Display: The screen of smoke detector carbon monoxide detector combo will display the CO or smoke concentration and illuminate when it goes off. This