Kidde KN-COEG-3 Plug-In Combination Carbon Monoxide Detector and Gas Alarm with Bonus Ionization Smoke Alarm Battery Backup Combination carbon monoxide and explosive gas alarm includes a 9 Volt battery backup Equipped with a peak level button, pressing the button you can see the peak CO level recorded by the alarm since it was last reset or unplugged Continuous digital display shows you the level of carbon monoxide (if any) the unit is sensing as well as if gas is present