A method for servicing, especially checking, an apparatus for capacitive ascertaining and/or monitoring at least one process variable of a medium, wherein the apparatus has at least one probe unit with a probe electrode and an auxiliary electrode. The probe electrode and/or the auxiliary electrode are/is supplied at least with one test signal or connected with at least one electrical potential, that at least one response signal is tapped from the probe unit, and that, at least from the response signal and a predeterminable desired value, information is obtained concerning the apparatus.