lets start from the beginning, does the wheel turn freely with the motor in place. if you pushed the unit would it roll? if the answer to both of these questions is yes and the motor is still out replace the speed sensor. it is the bearing with the wires on it. it is possible but difficult to test out of the unit and too cheap to mess with. once you have determined that this is not mechanical, reinstall the motor with the new bearing and test the unit. if problem is still present swap the two controllers on the left side of the truck (sitting on it) if it is a controller the problem will change sides.