Just a heads up, the EcoBee doesn’t use a CO2 sensor, it just guesses what the value is. I have the same thermostat and was getting wildly different readings than my more expensive AirThings View Plus. Checked the EcoBee support website[1] and it made a lot more sense.
“Based on the VOC measurement, it can also estimate carbon dioxide (CO2) levels. The sensor doesn’t directly measure CO2 but assesses CO2 levels based on the average correlation between VOCs and CO2 in exhaled human breath.”
[1] https://support.ecobee.com/s/articles/air-quality-sensor-faq...
Which seems fair, because I live in a normal home with breathing humans and not an industrial setting where VOCs are produced in other ways.
How wildly off were the readings btw? And were you doing things related to VOCs? Which reading was erroneously under reported compared to actual, CO2 or VOC?