First Alert Battery-Operated CO Detector senses and warns you of deadly carbon monoxide...and its dying battery Carbon monoxide (CO) is a deadly, colorless, odorless gas created by faulty gas heaters, furnaces or automobile exhaust. But because it's odorless, without plug-in or battery-operated CO detectors there's no way you could know if deadly carbon monoxide was seeping into the air you breathe. Our First Alert battery operated CO detector senses that gas, and sounds a loud 85dB alarm to warn you and your family. Wall-mounted CO detector sits in its mounting bracket included. Also includes two silence features, test button and batteries. Audible and visual low battery signal alerts you to replace the battery. Listed to UL 2034 standard.