This is an index, not a repair manual. Each entry is a symptom, the usual causes, and the parts most often involved. It exists because searching "my e-moto won't turn on" tends to return forum threads and nothing that tells you where to start.
Every fault below has its own page with the relevant parts listed. Work through the cheap checks before replacing anything: a surprising number of these end at a loose connector.
Won't start / completely dead
Usually a dead battery, a blown fuse on the controller, or a disconnected display. Involves battery, controller and display.
Battery drains fast / short range
Cells degrade over time. Check cell voltage under load — if any cell is below 3V the pack needs replacing. Involves the battery.
Won't charge / charging port issue
Check the charge port for bent pins. If the charger light never turns green, the BMS inside the battery may have tripped. Involves battery and controller.
Motor grinding or clicking
Grinding usually means worn motor bearings. Clicking is often the chain or sprocket. Check chain tension first. Involves motor, drivetrain and chain.
Loss of power / feels slow
Common causes are battery voltage sag under load from weak cells, or the controller throttle-limiting because of a fault code. Involves motor, battery and controller.
Brakes feel weak or spongy
Hydraulic brakes need bleeding if spongy. Cable brakes need the cable tensioned, or the pads replaced if they are glazed. Involves the brakes.
Brakes squeaking or rubbing
Squeaking after wet riding usually resolves itself. Persistent squeal means contaminated pads. Rubbing means a misaligned caliper. Involves the brakes.
Suspension too stiff, soft, or bottoming out
Adjust preload and rebound first. If the fork seals are leaking oil it is time to rebuild or replace. Involves forks and rear suspension.
Chain slipping / drivetrain noise
Check chain stretch with a ruler — replace if 12 links measure more than 15.24 cm. A worn chain also eats sprockets, so replacing it late costs more than replacing it early. Involves chain and drivetrain.
Display showing error codes
Error codes E01 to E08 usually point to sensor or motor faults; consult your controller manual. A dead display can also mean a loose connector. Involves display and controller.
Lights not working
Check the headlight fuse and the wiring loom connector. Some controllers have a separate light output that can burn out. Involves headlight and controller.
Handlebar wobble or feels loose
Tighten stem bolts in a star pattern to the manufacturer torque spec. Also check headset bearings — any play means replace. Involves handlebar and forks.
Seat broken, cracked, or uncomfortable
A ripped seat cover lets water into the foam. Replace early, before the base cracks. Involves the seat.
Wheels vibrating or wobbling
Spin the wheel and watch the rim against a fixed point. More than 2 mm of wobble means the wheel needs truing or replacing. Involves front and rear wheels.
The pattern worth noticing
The controller appears in five of these fourteen entries — more than any other component. That is not because controllers fail often; it is because the controller sits between the battery, the motor, the display and the lights, so a fault there shows up as a symptom somewhere else. If you have two unrelated-looking problems at once, the controller and its connectors are the first thing worth checking.
The second pattern is that connectors account for a large share of the cheap fixes across the list. Before ordering a replacement part for any of these, unplug and reseat the relevant connector.