Error 23 on the 850C means “Head light error.” Most likely cause: a damaged light, a pinched light cable, or water in the light plug. Try this first: switch the lights off.
On bikes where the controller powers the lights, a damaged light, a pinched light cable or water in the light plug can cause a fault the controller detects. The bike may still ride, but get it fixed, because a short in the light circuit can damage the controller.
Step-by-step checklist
Work from the top. Steps go from free and easy to shop jobs. Switch the bike off before touching any connector. Tick each step as you go. Your progress is saved on this device.
Parts you might need
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Only buy a part if the free steps didn't fix it. Always match the voltage and connector type to your bike.
E-bike headlight
Match the light voltage your controller supplies. Your manual lists it.
Link coming soon
Electrical contact cleaner spray
Cleans dirt and corrosion from connector pins and battery contacts.
Link coming soon
Dielectric grease
A thin coat on connector seals helps keep water out after you clean them.
When it's time for a shop: if you've worked through every step and the problem is still there, take the bike to an e-bike shop. Bring your notes on when it happens, plus the code and your error history.