If the carbon dioxide (CO₂) reading is clearly above 430 ppm even immediately after intensive ventilation, it is possible that the CO₂ sensor is incorrectly calibrated. An incorrect calibration can lead to a shift of the zero line in rare cases due to various effects, e.g. climatic shocks, which should normally be corrected again by the automatic calibration within a short time. If this is recognizably no longer the case after a longer period of time, because the zero line (here 420 to 450 ppm atmospheric concentration, depending on geography) is no longer reached even with long and strong ventilation, this can be corrected by a manual calibration.
No cross-sensitivities are known with the carbon dioxide sensor, except for the shock sensitivity already mentioned above.