An apparatus and method for diagnosing damage to drive-train hardware of a vehicle in which a disconnector may obtain speed of an auxiliary drive wheel motor of the vehicle and speed of a main drive wheel motor of the vehicle and determines damage to the drive-train hardware including the disconnector that controls whether to interrupt delivery of power through engagement or disengagement between auxiliary drive wheel motor-side drive-train hardware and auxiliary drive wheel-side drive-train hardware, based on whether a difference between the speed of the auxiliary drive wheel motor and the speed of the main drive wheel motor exceeds a threshold value, Accordingly, preventing divergence of motor RPM by diagnosing damage to the drive-train hardware of the vehicle.