A diagnostic method, in particular to determine the leak tightness of at least one seal and/or one valve, for at least one component of a vehicle, wherein a piston cylinder unit, whose piston is driven by an electromotive drive, forms a pressure supply unit, wherein a control unit identifies the piston position and/or piston movement of the piston by means of at least one sensor and the pressure generated by the pressure supply unit or the motor current flowing through the drive by means of at least one sensor. During the diagnostic method, the control unit may be used to measure quantities relating to piston movement/position, pressure, and/or pressure variation.