First Alert CO614 Plug-in Carbon Monoxide Alarm with 9V Battery Backup and Digital Display Plug-in carbon monoxide alarm with digital display and 9V battery backup meets most state and local codes for carbon monoxide alarms. Digital Display - shows visual confirmation of CO levels in PPM End of Life Signal provides audible notification alarms needs to be replaced Peak Level Recall Button - recalls the highest level of Carbon Monoxide detected Single Button Test/Silence Electrochemical Carbon Monoxide Sensor - the most accurate sensor available as compared to other sensing technologies. Two silence features - can silence the low-battery chirp for up to eight hours or temporarily silence an unwanted nuisance alarm