Without actually being "Hands-On", I doubt anyone here can answer your question since the problem could be either the disk with 8 precisely placed magnets on it that attaches to the motor shaft, or the disk with a magnet sensor that is held in close proximity to the magnet disk and sends out a pulse every time one of the magnets passes by it, or neither of them, or both of them, or the wiring, or the connectors, or how they are physically attached.
What constitutes the speed sensor
assembly is circled in
Red on the attached.
Some of the possible problems could be:
1. The screw fastening the magnet disk to motor shaft could be loose.
2. One or both of the retaining rings holding the magnet sensor in close proximity to the magnet disk might have jumped out of its groove.
3. Broken wires.
4. Dirty connections.
5. The magnets have been demagnetized, or physically damaged.
6. The Hall Effect multiplier has failed.
The most likely suspect is item 29 on the parts list, but first, I'd make sure everything is properly attached and in the proper position and all the wires and connections are good.
