Issue
Display shows error on screen “ERROR 21H” or “Error 21H Current”
Solution
Support message received from MATE .
Yes we will be sending you a controller to fix the error 21 that still remains.
We are working on a document and possibly a video this week that will be on the Mate Assistant to help with changing the controller … It is located in the frame in front of the seat tube where the battery connects to the bike.
It looks like we should start shipping warranty controller very soon from the new Hong Kong facility. I will try and keep you updated as best I can on that!
Thanks again for your patience it is much appreciated!
It looks similar to this one but not exactly
credits: Nico Cellier