For nearly all sensors, you need to do one of two things: - Make sure auto-calibration is disabled and manually calibrate the sensor once a year or so by takin

For nearly all sensors, you need to do one of two things: - Make sure auto-calibration is disabled and manually calibrate the sensor once a year or so by taking it out into fresh air and running its fresh-air calibration routine. - Leave auto-calibration enabled and make sure that the sensor is exposed directly to fresh air for at least an hour every single day. These sensors are first and foremost designed for use in HVAC systems in commercial buildings, which usually (always?) have an overnight fresh-air purge, so to keep them calibrated, the manufacturers just assume the sensor will always see fresh air at least once a day calibrate them based on that assumption. In a residential environment, that usually doesn't work. And FWIW, I've run a bunch of Sensirion SCD30 sensors for a few months and they worked quite well, with auto-calibration disabled. They're supported by ESPHome as well. Sensirion also recently released their SCD4x sensors [0] which measure CO2 with some kind of black magic [1] instead of the usual NDIR. The coolest thing is that the sensor is relatively tiny and built for SMD assembly. [0]: https://www.sensirion.com/en/environmental-sensors/carbon-dioxide-sensors/carbon-dioxide-sensor-scd4x/ https://www.sensirion.com/en/environmental-sensors/carbon-di... [1]: https://www.sensirion.com/en/about-us/company/pasensr-technology/ https://www.sensirion.com/en/about-us/company/pasensr-techno...