Sensor for determining a chemical element and method for controlling such a sensor. The invention relates to a sensor (1) for determining a chemical element or a chemical compound (CO2) in a supplied gaseous medium with a reaction area (9), on and/or in which the element or the compound brings about a chemical reaction that changes a characteristic parameter (for example the work function), Version 3* of the reaction area (9), a sensor element (5) that is placed in the sensor (1) to detect the change in the characteristic parameter, a signal output (11) at which an output signal (UPS) corresponding to the chemical reaction detected by the sensor element can be detected as the measured variable, and a reaction control device (SC, 12) with an electrode system (10, 13) for applying a control voltage to affect a reaction in the reaction area (9). To be able to use the sensor more variably and more reliably, it is proposed to produce a surface topology of such dimensions that during operation, a moisture film (16) of deposited ambient moisture continuously coats the surface area of both the electrode system and the reaction area. Possibilities in particular are correction of long-term changes from the influence of surrounding gases, checking probe functionality, and selective adjustment to measure given chemical variables.