Apparatus (1) and method for determining and/or monitoring at least one predefined filling level of a medium (2) in a container (2a) at least with a sensor unit (3) and an electronic unit (6), wherein the electronic unit (6) is configured to apply an excitation signal to the sensor unit (3) and to receive a reception signal from the sensor unit (3), and wherein the electronic unit (6) comprises at least one first computing unit (8a) which is configured to determine at least the predefined filling level from the reception signal in a normal operating mode (14), a second computing unit (8b) which is configured to make a statement relating to the state of the sensor unit (3) in a diagnostic operating mode (15), at least one buffer (9,9a,9b,…) which is configured to store the at least one statement relating to the state of the sensor unit (3) and is electrically connected to the first computing unit (8a) and to the second computing unit (8b), and at least one switching element (10a, 10b) which can be used to switch back and forth between the normal operating mode (14) and the diagnostic operating mode (15).