Detects both smoke and carbon monoxide in a single detector. The CO smoke alarm is battery operated for easy installation. The photoelectric sensor detects smoke produced by smoldering fires and minimizes false alarms from cooking smoke or shower steam. The electrochemical CO sensor helps to detect carbon monoxide leaks from multiple sources such as faulty appliances. Lights alert you to what the danger is, and a single button allows you to test the unit monthly or silence a false alarm.