Kidde KNCOB-DP2 Tamper Resistant Plug-In Carbon Monoxide Alarm with Battery Backup Tamper Resist Feature : Unit alerts you if it has been unplugged or has come loose from the outlet Two LEDs : Red - Illuminates when in alarm mode. Green  AC power is present, normal operation Test/Reset Button : Tests CO alarm circuit operation and allows you to immediately silence the alarm End of Life Alarm : Alerts user to replace CO alarm after 10-years of operation Provides peace of mind against the silent killer