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KT / Kunteng LCD error 21: Current abnormality
KT / Kunteng controllers with LCD3, LCD5, LCD8, LCD8H displays (generic hub-motor kits and budget e-bikes)Dealer neededDo not keep riding
The KT controller measured an implausible current: either current flowing at rest or a reading its sensor cannot make sense of. Assist is cut. This is usually inside the controller, but a shorted motor phase can trigger it too.
Likely causes
- Controller current-sense circuit or MOSFET damaged
- Motor phase wires shorted (wet connector, crushed cable)
- Controller ran above its rated current for too long
- Water inside the controller housing
How to fix KT / Kunteng LCD error 21
- Power cycle once to exclude a glitch
- Unplug the motor and switch on: if 21 disappears, check the motor connector and phase wires for shorts
- Inspect the controller for a burnt smell or discoloured connectors
- Replace the controller (KT controllers are cheap and widely available); take the motor to a shop if the phases are shorted
Go to a dealer if: Go to a shop if the code stays with the motor unplugged and you do not want to swap the controller yourself.
Warranty: Budget kits and bikes usually have 1 year of seller warranty; contact the seller before opening the controller or motor.
Typical cost: A KT controller is EUR/USD 40-90; a hub motor with burnt phases EUR/USD 150-300.
Warranty: Budget kits and bikes usually have 1 year of seller warranty; contact the seller before opening the controller or motor.
Typical cost: A KT controller is EUR/USD 40-90; a hub motor with burnt phases EUR/USD 150-300.
Still stuck? Take a photo of the display and show it to a local e-bike workshop. Bring the bike's system name (e.g. Bosch Performance Line CX) and the exact code; that saves diagnostic time and money.
Frequently asked
Can I reset KT error 21?
A power cycle clears a one-off. A repeating 21 is hardware.
Can I keep riding with error 21?
No, the controller blocks power and a phase short can make it worse.
Warranty?
Yes for the controller within the seller's warranty; not if it was reprogrammed to higher current.
Related KT / Kunteng LCD codes
Not an official KT / Kunteng LCD document. Compiled from workshop experience and public service information; details can differ per system version and bike brand.