Bafang All BBS/BBSHD — Error Code 21

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

Communication failure between display and motor controller

Hall sensor error inside the motor

Controller overheat protection activated

Always disconnect the battery before inspecting wiring.