First Alert Carbon Minoxide Alarms 2 Pk.This handy 2 pack 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