F21 on a Bosch washing machine usually indicates a motor-related fault. Worn carbon brushes that can no longer complete the electrical circuit through the motor are the most commonly cited cause, though a blocked drum, a failed rotor position sensor or a faulty control board can also trigger it.
What F21 means
The motor circuit — brushes, Hall/rotor sensor, tachometer signal and control board — works together to drive and monitor the drum. F21 signals that this circuit is not behaving as expected, most often because the physical connection between brush and motor commutator has worn away.
Common causes
| Cause | How likely | Notes |
|---|---|---|
| Worn carbon motor brushes | Common | Most frequently cited cause; brushes wear down over years of use |
| Blocked or obstructed drum | Occasional | Something preventing the drum from turning freely |
| Failed Hall / rotor position sensor | Occasional | Control module isn’t receiving engine speed information |
| Faulty tachometer signal | Less common | Similar symptom to a sensor fault |
| Faulty control board | Less common | Other causes ruled out |
What you can safely check
- Power cycle the machine
Unplug it for a minute and try again — this clears a latched fault in some cases without touching anything internal.
- Check the drum turns freely by hand
With the machine unplugged and empty, turn the drum. It should move smoothly with no grinding or resistance. Remove any obstruction you can see or feel.
Diagnosing worn carbon brushes, a rotor sensor or the control board means opening the motor housing and working with mains wiring. Technicians strongly advise disconnecting the machine from the power supply and calling a specialist rather than investigating powered equipment yourself.
When to call a professional
Beyond a power cycle and confirming the drum is not physically obstructed, F21 is motor and control-board diagnostic work. Have a technician check the carbon brushes, the rotor position sensor and the tachometer signal.
Stop and call a qualified appliance technician or electrician if you find scorched or melted wiring, smell burning, see water that has reached electrical components, or if the fault returns immediately after a repair. Never bypass a safety device — thermal fuses, door interlocks and flood switches fail for a reason, and defeating one converts a broken appliance into a fire or flood risk.
