Our CO detector claims to last seven years, the one that necessitated replacement by scaring the bejesus out of me at 5am one morning is still running too, I meant to throw it out, but it's still on the shelf of the kitchen as I found when I did some cleaning at the weekend (it's moving in the right direction, I think it stopped because I was unsure how one correctly disposes of CO detectors). I should throw it out but I assume it wasn't a plea for batteries but an actual false alarm (I'm alive and the new box gives a flatline unmeasurable level of CO, so I'm assuming it wasn't real and I spent fifteen minutes with my head out of the window for no good reason). Never looked how they work.