Hear the alarm everywhere: When a detector senses smoke or dangerous CO levels, all wirelessly connected units are activated to ensure the earliest possible warning. Wide coverage for complete home protection: Up to 24 X-Sense Link+ devices can be connected to create a wireless security network for your entire home. 2-in-1 protection: Equipped with a photoelectric smoke sensor and an electrochemical CO sensor for reliable protection against smoke and CO. Built-in 10-year battery: Features a built-in 10-year lithium battery, so you won't have to worry about changing the battery during the device's lifespan. Informative LCD: Displays real-time CO levels from 30-999 ppm (parts per million) from the surrounding air, as well as battery status and operational status.