1. A device for monitoring a voltage supply for a vehicle system, comprising: at least one comparison device which compares at least one of an output voltage of the voltage supply and a voltage derived from the output voltage as a first input signal to a predefined threshold voltage as a second input signal, and outputs a resulting comparison signal to a reset device which generates at least one reset signal as a function of the comparison signal; and means for performing a self-diagnostic function which (i) evaluates a logic state of the comparison signal which is output by the comparison device and (ii) evaluates a logic state of the reset signal, wherein the self-diagnostic function recognizes a malfunction of the comparison device when the evaluated logic state of the comparison signal does not match an expected logic state, and wherein the self-diagnostic function recognizes a malfunction of the reset device when the evaluated logic state of the reset signal does not match an expected logic state.