Read this if you want to save your time reading the too much info in the manual. Its overwhelming. This kind of stuff there should be a separate manual that features only the important instruction.- first and foremost 4 star only coz I wanted to buy the 3 pcs pack but good thing Ive read some comments with same complaints that they bought 3 pcs but they received only 1 pc. That must be a hassle and inconvenient to raise a complaint so I just bought 2 separate item and I am planning to buy another 2.- REASON WHY I BOUGHT THIS BECAUSE IVE READ: that First Alert is a reputable brand known for fire and CO safety products. Battery Backup: Works during power outages essential for 24/7 protection.Easy to install, no hardwiring needed.And the most important is it is “UL Certified” it Meets safety standards set by Underwriters Laboratories. Compared to other co detectors in amazon. These certifications mean the CO detector:•Has passed rigorous, standardized safety tests•Will reliably detect dangerous levels of carbon monoxide•Won’t give false alarms or miss actual dangerAnd when you buy this whether u like it or not you have to put the battery first and plug it. Coz if you dont put the battery and just plug it directly there will be a chirp sound every 30 seconds or so, and I found out thats an indication you need to install the battery. My bad I did not read the manual coz it overwhelms me a lot coz its too much to read for a supposed to be direct instructions only.So I tested my 2 first alert and it works. And you have to test it at least once a week. How I know this really works? I dont know I just have to trust the brand. And not wishing to have CO leak in our house. But this will bring us peace of mind knowing we have a CO detector at home esp so many sad cases about co leak in the house and people die. BE CAREFUL Y’ALL!