Bafang All BBS/BBSHD — Error Code 21
Reviewed by certified eBike technicians · Last updated July 30, 2026
Quick Answer
21 on Bafang = Speed sensor error — magnet misaligned or sensor faulty
Severity: Check soon — speed may be affected · Limited riding OK, diagnose soon
Error 21 on your Bafang indicates: Speed sensor error — magnet misaligned or sensor faulty.
Check soon — speed may be affected — Speed sensor error — magnet misaligned or sensor faulty
Symptoms
- Error 21 on display; speed shows 0 even when riding; motor may cut out intermittently
Common Causes
- Speed sensor magnet moved out of position
- Speed sensor not detecting wheel rotation
- Broken speed sensor wire
- Controller not receiving speed signal
Fix Steps
Step 1: 1. Check the speed sensor magnet on the rear wheel spoke — it should pass within 5mm of the sensor on the chainstay
Step 2: 2. Realign the magnet so it passes directly in front of the sensor
Step 3: 3. Check the sensor wire for cuts or pinch damage
Step 4: 4. Clean the sensor face with a dry cloth
Step 5: 5. If problem persists, replace the speed sensor (~$10 on Amazon)
Related Error Codes
Error 30
Communication failure between display and motor controller
Error 08Hall sensor error inside the motor
Error 30HController overheat protection activated
Error 07Over current protection — motor current exceeded safe threshold, controller triggered protection circuit
Always disconnect the battery before inspecting wiring.