This 3-pack of X-Sense SC07 smoke and carbon monoxide detectors is $16 off at Amazon. Each alarm has a sealed 10-year battery and an LCD that shows live CO levels. These are standalone units that do not interconnect wirelessly or support Wi-Fi. Shipping is free. Buy Now at Amazon
Features
Combination smoke and carbon monoxide alarm in one unit Sealed lithium battery rated for 10 years of power LCD display shows battery level and real-time CO readings UL 217 & UL 2034 certified Standalone unit that mounts on a wall or ceiling with included hardware Pack of 3 alarms; no rewiring required
