Bosch error 850: Fault in the rear wheel speed sensor cable
The ABS unit has lost the rear wheel speed sensor: open circuit or short in the cable or connector. The ABS warning lamp stays on and ABS is switched off, but the brakes work as normal brakes and the motor keeps assisting.
Likely causes
- Cable damaged along the chainstay
- Connector loose or corroded
- Sensor knocked or damaged by a chain or a stick
- Cable pinched during a wheel or tyre change
- Sensor failed internally
How to fix Bosch error 850
- Check the rear sensor plug and cable along the chainstay
- Push the connector home and check for damage
- Restart and test
- If the code persists, a dealer must test the circuit and replace the sensor or cable
Warranty: The ABS module is a Bosch component with the same 2-year warranty as the rest of the system; damaged sensor cables or bent sensor brackets from crashes are not covered.
Typical cost: A sensor or cable is about 30-60 EUR/USD plus labour.
Frequently asked
Can I reset Bosch error 850 myself?
Only by fixing the connection and restarting.
Can I keep riding with error 850?
Usually, but with reduced or interrupted assistance and the risk of the system shutting down mid-ride. Ride home carefully or on muscle power alone; do not plan long rides until the cause is found.
Is error 850 covered by warranty?
The ABS module is a Bosch component with the same 2-year warranty as the rest of the system; damaged sensor cables or bent sensor brackets from crashes are not covered.
Related Bosch codes
Not an official Bosch document. Compiled from workshop experience and public service information; details can differ per system version and bike brand.