ESPHome gives makers control over the reference used by two Sensirion CO₂ sensor families. Compatible SCD4x firmware can use a chosen automatic-calibration target instead of the fixed 400 ppm baseline; older firmware keeps the default.
SEN63C, SEN66 and SEN69C sensors gain a separate action to recalibrate against a reference concentration. Measurements must be stopped during that operation. The target-setting check used an SCD40, while forced recalibration was tested on a SEN66.