Universal Security Instruments CD-9690 3-LED Plug-In Carbon Monoxide Alarm with Digital Display and Peak CO Level Memory Battery backup continues protection, even during a power failure. Peak CO level memory button displays the highest recorded CO level The unit contains front-load battery drawer for easy access when it is time to replace the battery The 120-Volt AC direct plug in alarm is fast and easy to install Test buttons for alarm and power No costly sensor replacement. 3LED's: alarm power and service alarm condition indicators