Co detectors age out. Two of ours did so within days of eachother. The brand I have, despite having a voice, uses 5 beeps to tell you that. In the middle of the night, you just assume it's low battery and then get confused when it doesn't stop beeping with fresh batteries. (then you find the breaker, pull it's batteries, reset it and then look up on the Internet what it's trying to say, rather than reading the key on the back of the unit, but it's 5:30am and you aren't thinking so straight.)
Wired detetors will only talk to those of the same brand. This makes swapping out one of our 11 detectors for something nicer, like a nest, impossible.
Very grateful they still sell our detector. Hoping it comes soon, since the third is likely to age out soon.