Method for determining an oxygen concentration in a waste gas using a sensor element which has the following: - in its interior, a cavity which is connected to the waste gas via an access opening, - in its interior, furthermore a reference space which is separated from the cavity, - an electrochemical pump cell that has a first electrode, which is exposed to the waste gas on the outside of the sensor element, and that has a second electrode, which is arranged in the cavity, and that has a first solid electrolyte via which the first electrode is connected to the second electrode for the purpose of conducting oxygen ions, - an electrochemical Nernst cell that has a third electrode, which is arranged in the cavity, and that has a fourth electrode, which is arranged in the reference space, and that has a second solid electrolyte via which the third electrode is connected to the fourth electrode for the purpose of conducting oxygen ions, wherein the method makes provision for the oxygen concentration in the cavity to be regulated to a constant value by virtue of the fact that: - a Nernst voltage signal representing the Nernst voltage (Uvs) forming across the Nernst cell is detected, - at least one further variable (p) is detected or ascertained, namely: a pump current signal representing a current flow through the pump cell; and/or a pump cell signal representing a voltage across the pump cell, - a compensated signal (x) is ascertained from the Nernst voltage signal and from the at least one further variable (p), - a fault signal (e) is ascertained as the difference between a setpoint value (r) and the compensated signal (x), - the fault signal (e) is supplied as an input variable to a controller, - the output variable (u) of the controller is applied to the pump cell so that a current flow through the pump cell results, and wherein the method makes provision for a signal representing the current flow through the pump cell to be output as a measurement variable of the oxygen concentration in the waste gas. - in its interior, a cavity which is connected to the waste gas via an access opening, - in its interior, furthermore a reference space which is separated from the cavity, - an electrochemical pump cell that has a first electrode, which is exposed to the waste gas on the outside of the sensor element, and that has a second electrode, which is arranged in the cavity, and that has a first solid electrolyte via which the first electrode is connected to the second electrode for the purpose of conducting oxygen ions, - an electrochemical Nernst cell that has a third electrode, which is arranged in the cavity, and that has a fourth electrode, which is arranged in the reference space, and that has a second solid electrolyte via which the third electrode is connected to the fourth electrode for the purpose of conducting oxygen ions, wherein the method makes provision for the oxygen concentration in the cavity to be regulated to a constant value by virtue of the fact that: - a Nernst voltage signal representing the Nernst voltage (Uvs) forming across the Nernst cell is detected, - at least one further variable (p) is detected or ascertained, namely: a pump current signal representing a current flow through the pump cell; and/or a pump cell signal representing a voltage across the pump cell, - a compensated signal (x) is ascertained from the Nernst voltage signal and from the at least one further variable (p), - a fault signal (e) is ascertained as the difference between a setpoint value (r) and the compensated signal (x), - the fault signal (e) is supplied as an input variable to a controller, - the output variable (u) of the controller is applied to the pump cell so that a current flow through the pump cell results, - a Nernst voltage signal representing the Nernst voltage (Uvs) forming across the Nernst cell is detected, - at least one further variable (p) is detected or ascertained, namely: a pump current signal representing a current flow through the pump cell; and/or a pump cell signal representing a voltage across the pump cell, - a compensated signal (x) is ascertained from the Nernst voltage signal and from the at least one further variable (p), - a fault signal (e) is ascertained as the difference between a setpoint value (r) and the compensated signal (x), - the fault signal (e) is supplied as an input variable to a controller, - the output variable (u) of the controller is applied to the pump cell so that a current flow through the pump cell results, and wherein the method makes provision for a signal representing the current flow through the pump cell to be output as a measurement variable of the oxygen concentration in the waste gas.