There's a wire running from the speed sensor in the motor (often white), to the controller, that uses the information it gets to send serialized data to the display.
If everything else seems to be working OK, I'm thinking that the wire carrying the serialized data from the controller to the display is OK. That leaves the sensor in the motor, the wire from there to the controller, and the controller itself as possible culprits. My thought anyway, FWIW.