Sudden drops are resolved now as I've switched from breadboard to PCB. But it seems strange to me, that in closed empty room co2 have dropped by 300ppm over night. I have good windows and also humidity was constant whole night so I don't think the fresh air made it into a room

I still have it on breadboard, I've checked the output and it seems there are some restarts. Maybe it drops after restart, it is possible that first readings after sensor powerup are lower. I'll rebuild it to PCB

Here are my readings (green is the temperature).
I've set correct altitude and today I've still got readings around 200ppm:
there is also lot of sudden drops for short time (although home was empty for most of the day).
I'll try to bring it outside when I'll have more time.

I've found out there is altitude or pressure settings, so I've ordered barometric pressure sensor and will play with it later. Also when I compare data with SI7021, the temperature is off by 2 degrees and humidity by 10%

guys how accurate readings are getting you from this sensor? I've received it yesterday and I'm trying it but it gives suspicious values. When I open the window it drops as low as 210 ppm (maybe even lower but I'm sampling at 1 min rate) which is nonsense and then slowly climbs up after few minutes