Error 21 on Rad Power Bikes LCD and LED Displays

In plain English

The controller limits how much current flows to the motor. This code means it saw something unusual: a spike, a short in the motor wiring, or a motor working far too hard. Damaged motor cables, wet connectors and dragging wheels are common causes. Sometimes it's an internal controller fault.

Work from the top. Steps go from free and easy to shop jobs. Switch the bike off before touching any connector. Tick each step as you go. Your progress is saved on this device.

As an Amazon Associate I earn from qualifying purchases. Steps marked "affiliate link" link to Amazon.

1. 1Turn it off and on againFree Switch the display off, wait 30 seconds, then switch it back on. If the code clears, note what you were doing when it appeared, in case it comes back.
2. 2Check that the wheels spin freelyFree Lift each wheel and spin it by hand. Grinding, dragging or a rubbing brake makes the motor work harder.
3. 3Look over the motor cableFree Follow the cable from the motor along the frame. On hub motors, look closely where it leaves the axle: it often gets pinched when the wheel is removed or crushed when the bike falls. Look for cuts, kinks or bare wire.
4. 4Let it cool downFree If it happened after a long climb or hard riding, switch off and let the bike cool before riding again.
5. 5Unplug and check the motor connectorEasy Switch off and remove the battery. Follow the motor cable to its plug (often near the rear axle or under the frame). Pull it apart, look for bent or pushed-back pins, water, corrosion or burn marks. Burn marks mean stop and see a shop. Clean corrosion with electrical contact cleaner and let it dry fully, then push it back together firmly.

   Need it? Electrical contact cleaner spray on Amazon (affiliate link)

6. 6Dry the bike out if it got wetModerate If the problem started after rain or washing, remove the battery and leave the bike somewhere warm and dry for 24 to 48 hours, then test again. Don't use a heat gun or put the bike next to a heater.
7. 7Have the controller testedShop job If the wiring checks out, the controller may be faulty. A shop can test it and replace it if needed.

Parts you might need

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Only buy a part if the free steps didn't fix it. Always match the voltage and connector type to your bike.

• Electrical contact cleaner spray

  Cleans dirt and corrosion from connector pins and battery contacts.

  View on Amazon

• Dielectric grease

  A thin coat on connector seals helps keep water out after you clean them.

  View on Amazon

• Motor extension cable

  Replaces a damaged section of motor cable. Must match your connector and pin count.

  Match your bike's model first. Ask a shop if unsure.