The First Alert BRK SC-9120B Hardwired Combination Smoke and Carbon Monoxide Alarm uses electrochemical CO sensing technology and Ionization sensor for smoke detection. This hardwired dual CO smoke alarm can be connected to other compatible BRK or First Alert hardwired CO smoke detectors within a single-family residence. The latching alarm indicator automatically identifies which unit initiated the alarm--even after the conditions that set off the alarm have subsided. Using a 9-volt backup battery, the smoke and carbon monoxide detector can function even in the event of a power outage. Additionally, the CO smoke alarm features a single test and silence button, as well as indicator lights that let you know whether the danger is smoke or carbon monoxide. The tamper-resistant locking brackets and universal mounting brackets make installation easy.