A method for operating a drive device of a motor vehicle which has at least one electric machine with a phase angle detector and an output shaft operatively connectable with the electric machine, includes: when a return value of the phase angle detector of the electric machine is constant, performing a first diagnostic operating mode of the drive device, in which the operative connection between the electric machine and the output shaft is interrupted, subsequent, the electric machine is controlled by open lop control to a defined rotational speed, and when the return value continues to be constant after controlling the electric machine to the defined rotational speed, a first operating state of the phase angle detector is recognized and otherwise a correct function of the phase angle detector is recognized.