Did you ever get this figured out?

Hi everyone, hoping to get some advice from anyone who's dealt with this before.

I recently picked up a RadRover 6+ used and it's throwing Error 21 on startup. The display boots normally through the RAD sequence, then once it hits the speed screen it flips to an oscillating 21. The bike will not assist.

Here's what I've already tested to rule things out:
I unplugged the tail light and the error stayed.
I unplugged both brake cutoff switches and the error stayed.
I swapped in a known good display from another working RadRover 6+ and the error stayed.

One brake-related issue I still have is that only the right brake lever activates the rear brake light. The left lever does not, even though the front brake itself works mechanically. The rear brake is weak and likely needs a bleed, but I understand the hydraulic issue is separate from the electrical brake cutoff.

At this point I'm trying to determine if this is a controller issue, wiring harness issue, or something less common like the motor. I do have another working RR6+ and I'm planning to try swapping components to isolate the fault.

If anyone has had Error 21 persist after ruling out the display, brake switches, and tail light, I'd really appreciate hearing what the actual fix ended up being.

Thanks in advance for any insight. I'm trying to avoid throwing parts at it if possible.