A power steering system for a motor vehicle comprises a speed sensor 11, a steering torque sensor 12, a current calculating/driving circuit 21 for outputting a motor current command value on the basis of the vehicle speed and the steering torque, an assist torque generating electric motor 10 coupled to the steering system through an electromagnetic clutch 9, a clutch control circuit 50 for controlling a coupling effort of the electromagnetic clutch, an abnormal torque decision calculating circuit 31 for arithmetically determining an abnormal torque and a vehicle speed corresponding to the abnormal torque for a predetermined reference temporal duration to thereby output both an abnormal torque decision value indicating an abnormal torque corresponding to the vehicle speed detected by the vehicle speed sensor and the reference temporal duration, a load torque arithmetic circuit 33 for arithmetically determining both a load torque value indicating a sum of the steering torque detected by the steering torque sensor and an assist torque generated by the electric motor and a temporal duration of the load torque value, a torque comparison circuit 35 for comparing the load torque value with the abnormal torque decision reference value and comparing the temporal duration of the load torque value with the reference temporal duration, and a motor fault monitor/control circuit 40 responsive to results of the comparisons indicating that the load torque value is greater than the abnormal torque decision value and that the temporal duration of the load torque value is not shorter than the reference temporal duration, to thereby control the clutch control circuit so that the coupling effort of the electromagnetic clutch is lowered in a continuous manner to a predetermined coupling effort which permits manual steering.