Speed sensor fault

Stop riding Last verified: pending review Reviewed by: editorial team

Quick answer

Error code 21 signifies a fault with the speed sensor, which measures the wheel speed for the e-bike's controller. When this error occurs, the motor will not operate, and the display will show the error code. The issue is typically related to mechanical problems such as a misaligned magnet or a damaged cable.

Stop riding Sensor Often DIY-friendly Beginner (15–30 min)

Does this apply to your bike?

H700 · Bafang · Common displays: DP E171

Heads-up: the same number can mean different things on other systems — always check your drive system first. Browse all drive systems →

What you’ll notice

The display shows error code 21, and the motor is non-functional.

Likely causes, ranked

Start at #1 — causes are ordered by how often they turn out to be the problem.

#1 Misaligned or Missing Magnet

How to check: Inspect the spoke magnet to ensure it is properly aligned and not missing.

#2 Loose or Disconnected Sensor Connector

How to check: Check the sensor connector for a secure fit and proper seating.

#3 Damaged Sensor Cable

How to check: Examine the sensor cable for any visible damage or pinching.

#4 Failed Speed Sensor

How to check: Test the speed sensor functionality by checking if the LED blinks when the magnet passes.

Troubleshooting steps

Restart the System

easy

Turn off the e-bike and then turn it back on to see if the error clears.

If it passes →Check the magnet position.

If it fails →Proceed to check the speed sensor.
Check Magnet Position

easy

Ensure the spoke magnet is aligned correctly and within 10-20mm of the sensor.

If it passes →Check the sensor connector.

If it fails →Adjust the magnet position.
Inspect Sensor Connector

easy

Look for loose connections or corrosion on the sensor connector.

If it passes →Check the sensor cable.

If it fails →Clean or repair the connector.
Examine Sensor Cable

intermediate

Inspect the cable for any signs of damage or wear.

If it passes →Test the sensor functionality.

If it fails →Replace the sensor cable.
Test Speed Sensor Functionality

easy

Spin the wheel and observe if the sensor's LED blinks.

If it passes →Error resolved.

If it fails →Replace the speed sensor.

When to stop and get professional help

If the error persists after all checks and replacements, consult a professional.

Never open, puncture, or disassemble the battery pack, and never bypass the BMS. See our safety guide.

More codes for H700

13 · Battery temperature sensor fault 14 · Overtemperature of controller 15 · Controller temperature sensor fault 25 · Torque signal fault 26 · Torque sensor speed signal fault 27 · Over current of the controller

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Sources & verification

Last verified: pending review Reviewed by: editorial team

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