You didn’t quantify by saying you have it tested other than with the test button, as said before this doesn’t test the CO sensor just the electronics, bit like saying there is nothing wrong with my caravan battery as it shows 13v, it doesn’t prove anything until it’s tested correctly. And actually I do have detectagas, I use it on the house CO alarms fairly regularly 😁
I didn’t say all Kiddie ones fail, and I didn’t say yours did fail either, but I know that a lot of them (Kiddie ones) do fail, as it happens I know a lot of AWS engineers that comment on them.
A lot of the Kiddie Co alarms with go partial alarm as in pip every so often when tested, but don’t go to full alarm. The Fire Angel variants don’t seem to have the same issue.