This handy 2pk of First Alert CO400 Carbon Monoxide Alarms utilizes electrochemical carbon monoxide sensors to monitor your space for carbon monoxide threats. The battery-powered design doesn't require an electrician; simply insert the included 9-volt battery into each alarm, and the carbon monoxide detectors are ready to go. The advanced electrochemical CO sensors detect carbon monoxide leaks from multiple sources, such as faulty fuel-burning appliances, blocked chimneys, and generators running inside or too close to homes. When CO levels become elevated, a loud 85-decibel alarm alerts you to the threat. Because CO evenly distributes itself, it's important to have a CO detector on every level and in every bedroom of your home. Designed to be simple to use, each CO detector has just one button that silences or tests the unit, as well as an end-of-life timer that lets you know when it's time to replace the device.