ONE UNIT - First Alert 2-in-1 Smoke and Carbon Monoxide Alarm Model PC1210 Listing is for ONE First Alert Smoke and Carbon Monoxide Alarm as shown in the photos. Original package was for TWO UNITS - This package only has ONE. Please refer to photos. Patented smoke entry system reduces false alarms No low battery chirps or battery replacements Photoelectric smoke sensor and an electrochemical CO sensor Built-in 10-year battery offers continuous power for the life of the detector End-of-life warning to let you know when it’s time to replace