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Rad Power Bikes error 21: Speed sensor error
Rad Power Bikes with Bafang hub motor and Rad LCD display (RadRover, RadCity, RadWagon, RadRunner, RadMini)Fix at homeRide with caution
The controller gets no wheel speed signal. Speed shows 0, assist cuts out after a few seconds or is limited. On Rad bikes this is a spoke magnet and a sensor on the chainstay near the rear wheel, and the magnet moving is by far the most common cause.
Likely causes
- Spoke magnet rotated or slid away from the sensor
- Sensor knocked out of alignment or its cable pulled loose
- Water in the sensor connector
- Faulty sensor
How to fix Rad Power Bikes error 21
- Find the magnet on the rear wheel spokes and set it 2-5 mm from the sensor, facing it; tighten
- Check the sensor connector on the harness near the chainstay; dry and re-seat
- Spin the wheel with the display on; speed should appear
- Replace the sensor and magnet (Rad sells them; generic Bafang ones fit)
Go to a dealer if: Go to a shop if a new sensor with a correctly placed magnet still gives 21.
Warranty: Rad Power Bikes gives a 1-year limited warranty on electrical parts (2 years on some newer models and regions); contact Rad support with a video of the error before buying parts.
Typical cost: Sensor and magnet about USD 15-25; usually free by realigning the magnet.
Warranty: Rad Power Bikes gives a 1-year limited warranty on electrical parts (2 years on some newer models and regions); contact Rad support with a video of the error before buying parts.
Typical cost: Sensor and magnet about USD 15-25; usually free by realigning the magnet.
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
How do I reset Rad error 21?
Fix the magnet gap and power cycle.
Can I keep riding?
Yes safely; assist will be limited until fixed.
Warranty?
A moved magnet is not a defect; a failed sensor is covered.
Related Rad Power Bikes codes
Not an official Rad Power Bikes document. Compiled from workshop experience and public service information; details can differ per system version and bike brand.