King-Meter error 21: Abnormal current detected by the controller

King-Meter SW-LCD / SW900 displayDealer neededDo not keep riding

The controller monitors the current flowing from the battery to the motor and has flagged a reading outside its safe operating range. As a result, pedal assist and throttle drive are switched off, walk assist is disabled, and the speed reading may freeze until the fault clears.

Drive system	King-Meter SW-LCD / SW900 display
Severity	Dealer needed
Keep riding?	Do not keep riding
Typical cost	Budget around EUR 40 to 90 for connector repair and diagnostics, EUR 120 to 220 for a controller swap, and EUR 250 to 450 for a full motor replacement.

Likely causes

Damaged or pinched motor cable between frame and motor
Loose or corroded connector at the controller or motor
Internal motor winding short caused by heat or moisture
Water ingress inside the controller housing or motor plug
Failing controller current sensor or MOSFET stage

How to fix King-Meter error 21

Power the bike off and remove the battery before any inspection.
Trace the motor cable along its full run and look for chafing, cuts, or pinch points near the dropouts and swingarm.
Unplug the motor and controller connectors, inspect the pins for green corrosion or bending, then reseat them firmly.
Dry any moisture and confirm both connectors are fully clipped in.
Reinstall the battery, power on, and check whether the code clears.
If error 21 returns immediately, the fault is inside the motor or controller electronics and cannot be confirmed without a current clamp or bench test.
Take the bike to a dealer for controller diagnostics and motor testing.

Go to a dealer if: A dealer is needed when the code reappears after the cables and connectors have been checked, because confirming whether the motor or controller is at fault requires specialist current measurement.
Warranty: A genuine controller or motor fault inside the two-year warranty window is normally covered, while water ingress, crash damage, and obviously worn cable insulation are treated as owner-side issues.
Typical cost: Budget around EUR 40 to 90 for connector repair and diagnostics, EUR 120 to 220 for a controller swap, and EUR 250 to 450 for a full motor replacement.

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 King-Meter error 21?

Switching the system off and back on can clear a momentary current spike, but a persistent abnormal current reading returns immediately and points to a real hardware fault that needs investigation rather than another reset.

Is it safe to keep riding with error 21 showing?

No. The controller has cut drive to protect the electronics, and pushing on without assist is unsafe in traffic. Forcing the bike to run can also stress the cabling and worsen the underlying fault.

How much will the repair cost and is it covered by warranty?

Dealer work typically runs EUR 120 to 450 depending on whether the controller or motor is replaced. Bikes under two years old with no sign of water or crash damage are usually covered for the faulty part.

Related King-Meter codes

22 · Throttle signal out of range 23 · Motor phase cable fault 24 · Motor hall sensor fault on the rear motor 30 · Display lost link to controller

Official source: King-Meter SW-LCD user guide, section 6.9 Error Code Display

Not an official King-Meter document. Compiled from workshop experience and public service information; details can differ per system version and bike brand.