If you have the manual it gives simple steps to trouble shoot the steps. If not you can find theses steps on line. It will test the motor, the magnetic switch in the door and the controller to determine which one is bad. Sounds like you kind of tested the motor and it works with no load and good voltage. Do you have good voltage at the plug that goes to the motor? Will the motor work when it is installed and you help the steps with your hand? With the motor out can you move the steps in and out with your hand without binding?