I have similar problem. I know the sensor should run for 20 minutes before getting correct values from it but mine runs for ~10 hours (I use this sketch: https://github.com/sparkfun/SparkFun_CCS811_Arduino_Library/blob/master/examples/BasicReadings/BasicReadings.ino) and it reports ~5000ppm CO2 in a small room where a windows is permanently opened. This can't be true.
I know it was not ran for 48 hours as advised as a burn in period. But it is not clear to me what should be considered as burn in process. Should I kept running above sketch for 2 days? Or is it necessary to leave it just powered on (without any sketch)?
I read it has some auto baseline correction which will be applied after 24 hours ("The air quality is expected to vary in a typical environment so the minimum time over which a baseline correction is applied is 24 hours." - https://cdn.sparkfun.com/assets/learn_tutorials/1/4/3/CCS811_Datasheet-DS000459.pdf)
Will it stabilize in time? Anyone got reliable accuracy from that sensor?