Method for determining an internal resistance of a sensor element of a sensor for detecting at least one property of a measurement gas in a measurement gas space, wherein the property of the measurement gas is detectable by the sensor element, wherein the sensor element comprises at least one first electrode, at least one second electrode and at least one solid electrolyte connecting the first electrode and the second electrode, wherein the sensor has at least one electronic control unit, wherein the method has the following steps: d) setting an operating state, wherein a heating voltage is applied to the sensor element, and wherein a substantially constant voltage state is set and detected at the sensor element; e) carrying out at least one diagnosis sequence, wherein at least one first diagnosis state is set by a diagnosis current being applied to the sensor element, wherein at least one second diagnosis state is set in which the diagnosis current is switched off, wherein at least one first voltage value is detected in the first diagnosis state and at least one second voltage value is detected in the second diagnosis state, and wherein the first voltage measurement value is determined during a settling process resulting from the diagnosis current being applied to the sensor element, characterized in that the second voltage measurement value is determined during a decay process resulting from the diagnosis current being switched off; f) determining the internal resistance R internal of the sensor element from the relationship R internal [ i] = ( U [ i] + U[i - 1] - 2 U offset)/I, where i is an iteration step, U [i] and U [ i - 1] are the first voltage value and the second voltage value, a diagnosis current I is applied to the sensor element, and U offset is a voltage offset in the operating state corresponding to the constant voltage state in the operating state. d) setting an operating state, wherein a heating voltage is applied to the sensor element, and wherein a substantially constant voltage state is set and detected at the sensor element; e) carrying out at least one diagnosis sequence, wherein at least one first diagnosis state is set by a diagnosis current being applied to the sensor element, wherein at least one second diagnosis state is set in which the diagnosis current is switched off, wherein at least one first voltage value is detected in the first diagnosis state and at least one second voltage value is detected in the second diagnosis state, and wherein the first voltage measurement value is determined during a settling process resulting from the diagnosis current being applied to the sensor element, characterized in that the second voltage measurement value is determined during a decay process resulting from the diagnosis current being switched off; f) determining the internal resistance R internal of the sensor element from the relationship R internal [ i] = ( U [ i] + U[i - 1] - 2 U offset)/I, where i is an iteration step, U [i] and U [ i - 1] are the first voltage value and the second voltage value, a diagnosis current I is applied to the sensor element, and U offset is a voltage offset in the operating state corresponding to the constant voltage state in the operating state.