Bosch error 840: Implausible signals from the rear wheel speed sensor
The ABS unit is receiving rear wheel speed signals that do not fit the front wheel signal or the motor's speed data. 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
- Rear sensor ring dirty, damaged or bent
- Rear wheel not seated correctly after a puncture repair
- Sensor air gap too large or sensor loose
- Wrong rear disc or ring fitted
- Rear wheel locking or slipping repeatedly on loose ground
How to fix Bosch error 840
- Check the rear wheel is seated and the axle tight
- Clean the rear sensor ring and sensor face
- Check the sensor gap and mounting
- Restart and ride a few hundred metres
- If the code persists, a dealer must check the sensor and read live data with DiagnosticTool
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: Usually free; a sensor is 30-60 EUR/USD, a sensor-ring disc 40-80 EUR/USD, plus labour.
Frequently asked
Can I reset Bosch error 840 myself?
Yes, by correcting the wheel or sensor and restarting.
Can I keep riding with error 840?
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 840 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.