I have several CO2 monitors including some from AirThings, Aranet4, and a industrial sensor in the form of a Vaisala GMP252 CO2 probe. They all track closely enough that I don't worry about it. The Vaisala is rated to +/- 40PPM which is accurate enough for living space environmentals.

I don't have any equipment to calibrate it or measure it's calibration against. However, when I leave the windows open for extended periods of time, the CO2 levels hit levels that are within 50ppm of [0]. (I live in a different city from this station, with different population density, traffic patterns and all sorts of other micro-climate stuff that can contribute to a baseline difference. Additionally, CO2 levels swing by several hundred ppm when the air quality goes bad, and the ~50ppm range is insignificant in comparison)

It seems good enough for how I use it. I wouldn't conduct chemistry experiments using this as an instrument.

[0] https://gml.noaa.gov/ccgg/trends/monthly.html