850C (Bafang) code list · 850C Display

Error 21 on 850C Display

What it means + how to fix it From the manual

Quick answer

Error 21 on the 850C means “Speed sensor error.” Most likely cause: a speed sensor magnet that has moved or fallen off, or a damaged sensor cable. Try this first: turn it off and on again.

Try this first

Free, and each takes under 5 minutes.

Turn it off and on again
Check the speed sensor magnet
Check the wheel is seated properly

In plain English

The system isn't getting a proper speed signal. On mid-drive motors this usually comes from a magnet on a spoke passing a sensor on the frame. Hub motors often read speed inside the motor, so there the motor cable matters. The display may show 0 speed, and assist may cut out.

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.

1Turn it off and on againFree Switch the display off, wait 30 seconds, then switch it back on. If the code clears, note what you were doing when it appeared, in case it comes back.
2Check the speed sensor magnetFree Find the small magnet on a spoke (or on the brake rotor) and the sensor on the frame it passes. Make sure the magnet is still there, hasn't slid along the spoke, and passes close to the sensor. Your manual gives the exact gap.
3Check the wheel is seated properlyFree If the wheel was removed recently, make sure it's fully seated in the frame so the magnet passes the sensor at the same spot.
4Check the speed sensor cableEasy Switch off. Follow the thin cable from the speed sensor to its plug. It often runs near the chain or rear wheel, where it can get cut. Pull the plug apart and push it back together firmly.
5Unplug and check the motor connectorEasy Switch off and remove the battery. Follow the motor cable to its plug (often near the rear axle or under the frame). Pull it apart, look for bent or pushed-back pins, water, corrosion or burn marks, let it dry, then push it back together firmly.
6Have the sensor replacedShop job If the sensor is damaged or still faulty after these checks, a shop can test and replace it.

Parts you might need

As an Amazon Associate I earn from qualifying purchases. These links go straight to Amazon.

Only buy a part if the free steps didn't fix it. Always match the voltage and connector type to your bike.

Speed sensor and spoke magnet
Replaces a lost magnet or damaged speed sensor. Match your motor system.
Link coming soon
Electrical contact cleaner spray
Cleans dirt and corrosion from connector pins and battery contacts.
Link coming soon

Still not fixed?

Pages to try next:

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.

Words on this page: PAS (pedal-assist sensor)Speed sensor