Error Code 21 is a speed sensor fault. Check the location of the magnet on your rear wheel to ensure it is passing over the speed sensor at the correct location. If the magnet is missing, please contact us to arrange delivery of a replacement or visit a bike shop to purchase a speedometer magnet. If the magnet location does not resolve the issue you may need to replace the speed sensor. This speed sensor works with the Bafang Max and Bafang Ultra motors.