There are a lot of bike wheel sizes and types out there, so installation of the speed sensor and magnet can vary based on your specific wheel. Below are steps in order to help you resolve your Error 21 issue!

An error 21 is typically a simple fix and is due to improper installation and placement of the magnet in regards to the speed sensor. The magnet must be installed within 1-2mm of the speed sensor. Please reference the Installation Video on our website and follow the steps for installing the speed sensor and magnet within the proper placement.

The path of magnet travel must be directly perpendicular to the orientation of the speed sensor in order to properly operate.

Our recommendation is to install the sensor as far away from the center of the wheel as possible in order to help make sure that the magnet path of travel is as perpendicular to the speed as possible. In the image below the X represents where the speed sensor should not be installed (close to the center of the rim) while the green circle shows where the speed sensor should be installed (close to the outside of the rim).

The speed sensor cables are actually relatively fragile and should not be yanked or pulled on (especially when being installed on bikes that should have speed sensor extensions in order to have the necessary length to function properly). It is not recommended to install these cables under tension as it can result in the cable being pulled from the controller. The only way to fix this would be to replace the entire controller. The speed sensor cable communication connection has narrow holes however if you suspect that the cable may have been pulled too tight during installation or use you can test for voltage from the controller by testing the voltage across the terminals as outlined in the photos below.