The invention relates to a method for detecting an error in a generator unit which has an electric machine (100) with a rotor winding (110), a stator winding (120), and a rectifier (130) connected thereto, via which the electric machine (100) is connected to an onboard electrical system (150) of a motor vehicle, wherein a voltage of the onboard electrical system (150) is controlled to a target value via an excitation current (IE) through the rotor winding (110) of the electric machine (100), and a curve of a phase voltage (Uy) of the stator winding (120) is monitored, wherein an error in the generator unit is inferred when a change of the phase voltage (Uy) is detected and a voltage of the onboard electrical system (150) changing within a threshold value range (S) is detected.