If your sensors are able to do remote logging, you can do your own double blind test and cover up the displays for a week and log what you feel like the CO2 level might be, and then at the end of the week compare it to the logged levels.

Either way, you might consider an air exchanger - either commercial or homemade[0] that can efficiently move air from outside to inside, without much heat loss.

[0] https://www.loudawson.com/17884/how-to-build-air-cross-flow-...