2-in-1 Detection - Smoke & carbon monoxide detector combo alerts you to both smoke & fire events as well as poisonous, CO gas in a single device Photoelectric sensor detects visible fire particles associated with slow, smoldering fires, while the electrochemical sensor detects carbon monoxide An 85-decibel alarm sounds & a red LED indicates the smoke detector is sensing smoke or fire Lithium battery operated: eliminates the need for battery replacement & provides reliable detection, even during power outages Test-Hush button makes it easy to test the smoke detector and silence false alarms that occur from cooking or shower steam The alarm will automatically activate when it is attached to the mounting bracket, there are no pull tabs, no switches, everything is automatic At the end of alarm life, the unit will chirp, indicating the alarm is in need of replacement UL listed with a 10-Year limited manufacturer warranty on the alarm only (not a battery performance claim). Warranty begins when the alarm is powered on. If that date is unknown, it should be assumed that replacement is needed.