I don't have the specific version of CO2 detector you have, but just some info here. The CO2 detector I have has two led status lights that will help determine the cause for an alarm. Not sure if your has the same but something to investigate into.
On my TT , which we also bought used and it was 3 years old when we bought it, I had our CO2 detector go off a couple times one evening. I thought I was going to have to replace ours so I removed the mounting screws to take a look. Where ours was positioned, the back of our CO2 detector is exposed to our galley cabinet, and the previous owner had spill some dish soap back there. Since that wasn't cleaned up very well, if at all, it ate away at the CO2 detector wire shielding and started corroding one of the wires. There were only a couple of strands of one of the wires intact, ultimately getting very low voltage. I wasn't too versed in the CO2 detector at that time and didn't realize the led lights were indicating low voltage but it all made sense after I had a root cause analysis (RCA) on what was going on and why the CO2 detector alarmed. Of course it was an easy splice of the wires along with cleaning up that dish soap mess I wasn't aware of and no more issues.
Just a FYI that low voltage to your CO2 detector doesn't always mean your batteries are low. Test the wires going to the CO2 detector to make sure you are getting the voltage it should be getting. There could be a compromised wire somewhere.
