The calibration of O2 sensor has to be the first action whenever anything “strange” is going on with Oxygen.
If possible a two-point calibration should be performed.
Two-point calibration will be performed if ventilation is not running and O2 is available. It produces better results than the one-point calibration as it adjusts the Offset (at 21%) and the sensitivity of the cell.
One-point calibration is performed during ventilation or if O2 is not connected. It adjusts the offset only.