Error 21 / E021
Various Electric Bike
Severity: ModerateWhat Does This Error Mean?
Error 21 means the speed sensor is not detecting wheel rotation. The magnet may have moved away from the sensor, or the sensor cable is disconnected. Check the gap between the spoke magnet and the sensor on the rear wheel.
Affected Models
- Rad Power Bikes
- Lectric XP
- Aventon
- Bafang motor systems
- Most e-bikes with speed sensors
Common Causes
- Speed sensor magnet moved from its position on the spoke
- Sensor too far from the magnet
- Sensor cable disconnected
- Sensor failed
- Magnet fell off the spoke
How to Fix It
-
Check the spoke magnet position.
The small magnet should be on a spoke directly opposite the sensor. It should pass within 2-3mm of the sensor when the wheel turns.
-
Adjust the sensor gap.
The sensor should be within 2-3mm of the magnet. Adjust its position by loosening the mounting screw.
-
Check the sensor cable.
Follow the cable from the sensor to the controller. Make sure it is plugged in and not damaged.
-
Replace the magnet if it fell off.
The magnet attaches to a spoke with a zip tie or adhesive. Replacement magnets cost $2-$5.
Frequently Asked Questions
What does the speed sensor do?
The speed sensor tells the controller how fast the wheel is spinning. The controller uses this to manage pedal assist levels and enforce speed limits.
Can I ride without a working speed sensor?
On most e-bikes, no. The controller needs speed data to provide pedal assist. Without it, the motor may not engage.