ok, lets use some real world heuristics.
> "When your results are impossible, the measuring equipment is likely broken" investigate if the sensors may be at fault, rather than the actual CO2 level
> investigate if something is draining the CO2 faster than the system can replace it. Given the capacity of the system, this likely means either it's venting into space of there's a massive fire.
> given you're repair unit 25, and the system is not likely to have 25 separate faults (then again, it's not likely to have any faults, so maybe you should question your assumptions about what is likely?), you are presumably a backup unit. Investigate what happened to the previous unit(s) in case the system has been hacked and you may be at risk ofbeing infected thus preventing you from fixing the error.
> while the system is highly resilient to general damage, there might be airborne chemical that block some critical component all at once. most likely some kind of particle that mimics CO2, but react slightly different and thus may clog filters or show up on sensors unreliably. Get mass spectrography on samples of atmosphere, filters, sensors, and CO2 storage o check for such contaminants.