This First Alert SC9120B Hardwired Smoke and Carbon Monoxide Detector includes a 9-volt battery backup that continues to protect you in the event of a power failure. The alarm features electrochemical CO sensing technology, an ionization sensor for smoke detection, and an 85-decibel alarm. One button silences any nuisance alarms, and also tests detector functions, while lights indicate if the alarm detects smoke or carbon monoxide. The latching alarm indicator automatically identifies which unit initiated the alarm, even after the conditions that set off the alarm have subsided. And an end-of-life signal enhances your safety by letting you know when it's time to replace the unit. Easliy connect it to compatible BRK or First Alert alarms within a single-family residence; connecting will sound all the alarms together.