Did you check continuity on wires with an Ohmmeter from plug J6 on control board to plug J3 on motor speed control? That is first step. Pain in the you-know-what but necessary to identify or eliminate wires as problem. Without communication from the main control, the motor control board won't output correct signals to the motor, so it won't do any good to test that until communication error is cleared.
I am not aware of the factory providing any voltage test points for the main control board in this case. Once the wires are tested and deemed good, the next step is to replace the main control, as it is usually a processor error. In my experience, those go out more often than the motor control boards.