Checks for Carbon Dioxide (CO?) concentrations Max/Min CO? value recall function Automatic Baseline Calibration or manual calibration in fresh air Maintenance free NDIR (non-dispersive infrared) CO? sensor Indoor Air Quality displayed in ppm with Good (380 to 420ppm), Normal (<1000ppm), Poor (>1000ppm) indication User programmable High/Low Visible and Audible CO? warning alarms Displays Year, Month, Date, and Time