I suffered this problem with my Neo Cross for about 6 years. I check the continuity on the cables several times from motor to controller and controller to display to no avail.
After some inspiration I finally found a solution. I think something fails in the controller that really squashes the amplitude of the white wire speed signal. I checked on my oscilloscope and it's like 200mV. It should be 4.2V or more. I fixed it with a 220 ohm resistor pulling up that white wire to 5V at the connector between the motor and the controller at the controller end. It's happily worked for the past 100 miles.
I spliced it in to the wires carefully removing the insulation on the wires and soldering it in place. Then some hot melt glue and insulation tape to protect it.
You could of course just get a new controller. But for me, this fix was free.
(I used to change the assist level every 5 mins to stop the LCD going to sleep and still retain some level of assist. As there's no speed indicated the LCD thinks you are not cycling so automatically shuts off. By fiddling with the LCD it stops it falling asleep)
