Brose error 21: Thermal sensor fault in the drive unit
The motor controller has lost a reliable signal from its internal temperature sensor, which is what the drive uses to protect itself from overheating. As a safety response the system cuts motor assist and disables walk assist until the fault clears. The display still powers up and you can read the code, but the bike will not propel you under power.
| Drive system | Brose Drive (S, T, C - also used by Bulls, Rotwild, Fantic, BH, Trek Allant) |
| Severity | Dealer needed |
| Keep riding? | Do not keep riding |
| Typical cost | Diagnostic at a Brose dealer typically runs 40 to 80 EUR, and a thermal sensor or harness replacement inside the motor housing usually lands between 120 and 280 EUR including labour. |
Likely causes
- Damaged lead or connector on the internal thermistor inside the motor housing
- Corroded or unseated sensor plug at the motor controller
- Failed thermistor on the motor windings or controller PCB
- Water ingress reaching the sensor or its plug after heavy rain or pressure washing
- Wiring harness chafed where it routes near the motor mount or swingarm pivot
How to fix Brose error 21
- Stop riding and switch the system off using the control panel button.
- Leave the bike standing for two to three minutes so internal readings settle.
- Switch the system back on via the control panel and watch the display.
- If code 21 clears, ride normally and monitor whether it returns under load.
- If the code reappears, do not keep riding and check that no visible plug on the motor cable has come loose.
- Take the bike to an authorised Brose dealer for bench diagnostic and sensor or harness replacement.
Warranty: An internal sensor or wiring fault inside the drive unit is normally treated as a component defect and covered for two years from purchase, while water damage, crash impact and worn connectors outside the housing are usually excluded.
Typical cost: Diagnostic at a Brose dealer typically runs 40 to 80 EUR, and a thermal sensor or harness replacement inside the motor housing usually lands between 120 and 280 EUR including labour.
Frequently asked
Can I reset brose error 21?
Yes, start by switching the system fully off using the control panel button and then turning it back on. If the code reappears on the next ride, the sensor itself is likely at fault and a dealer reset will not hold.
Is it safe to keep riding with error 21?
No, because the drive cannot read its own temperature and will shut motor assist off without warning, sometimes mid-climb. Without a working thermal limit the controller also restricts power as a precaution, so ride home unpowered or arrange transport to a workshop.
What will the repair cost and is it covered under warranty?
Expect 120 to 280 EUR at most workshops including labour to open the motor housing and replace the sensor. A genuine sensor or harness fault inside the first two years is normally covered by the manufacturer warranty, but water ingress, crash damage or aftermarket modifications are not.
Related Brose codes
Manuals and video
Official source: Brose error code table, Pedego dealer service guide
Not an official Brose document. Compiled from workshop experience and public service information; details can differ per system version and bike brand.