First Alert CO605 Frust Free Carbon Monoxide Plug-in Alarm with Battery Backup 120-volt AC plug-in carbon monoxide alarm Mute button, mutes unnecessary alarms, also tests alarm function with same button EZ access battery door, front-open compartment allows easy battery change without dismounting alarm 9V battery included 7-year limited warranty, carbon monoxide alarms should be replaced every 5 years for maximum protection