Kidde 900-0235 Nighthawk Carbon Monoxide Alarm, Long Life AC Powered with Battery Backup Tamper resist feature unit goes into constant alarm mode if it has been unplugged or has come loose from the outlet 9V Battery backup provides continuous protection even during power outages Test/Reset button tests the unit for proper operation and resets the Carbon Monoxide alarm 7-Year end of life warning alerts user to replace CO alarm after 7-years of operation (40% longer than other CO alarms) Kiddes carbon monoxide alarms meet the requirements of Underwriters Laboratories (UL), an independent facility that tests products for compliance with widely-accepted safety standards