Abstract: A method and means for diagnosing transformer tap changers is provided. Pressure is monitored in an insulating liquid of a tap changer for a period of time in which switching of the tap changer is carried through. Switching-related information is extracted from the pressure diagram. Quantities from pressure peaks are related to specific arcings of the switching sequence. Early error detection is achieved, whereby transformed failures may be avoided. Repeated measurements may be followed by trend evaluation. If a trend indicating tap changer problems is detected, an alarm signal is triggered. The transformer may be switched off if the pressure exceeds a predetermined emergency value. Information of the base pressure in the tap changer may be extracted and converting it to level of insulating liquid.