Features memory scan which retains the CO level that activated the alarm. After ventilation, it is impossible to know what the CO level was unless your detector has a memory. The warning alarm notifies you of moderate levels of CO, helps protect high risk groups, seniors children. The CO detector has a long-life self-cleaning, maintenance free sensor. There's no need to replace the sensor every 2 or 3 years. The loud 85 DB alarm alerts you to CO danger and the test/reset button can be used to temporarily silence alarm during ventilation. Dimensions: Surface Mount: 5 1/2"W x 3"H x 1 1/3"D. Flush Mount: