A method for detecting a fault of an operating switch connected to an evaluating unit by at least one signal. The switch having at least a first switch circuit for initiating a vehicle function. A signal that specifies the switch position is sensed by the evaluating unit. The first switch circuit is connected to a first potential of the vehicle electrical system and to the signal line to form a first contact branch. The first switch circuit is bridged by a diode device in the non-conducting direction with respect to the first potential, the evaluating unit generates a test voltage, which is greater than the first potential, the test voltage is applied to the signal line, and, the potential being applied to the signal line of the first contact branch is evaluated by means of the evaluating unit in order to detect a fault.