Bosch F21
Motor/Control Error
High severityExpert Guide
SeverityHigh
What Your Machine Is Actually Telling You
F21 is one of Bosch's most common error codes and specifically means the motor didn't produce any rotational signal after the board applied power. The tachogenerator (speed sensor) reported zero RPM despite the board commanding the motor to spin.
F-series codes on Bosch indicate the most critical system failures. While E-codes are often sensor or peripheral issues, F-codes mean a primary function completely failed.
How the board diagnoses F21:
1. Board applies voltage to the motor.
2. Waits for tachogenerator signal (any speed reading).
3. After ~10 seconds with zero signal → F21.
The most common cause by far: carbon brushes. In the millions of Bosch washers with brushed motors installed in homes worldwide, worn carbon brushes account for roughly 40-50% of all F21 cases. The brushes are consumable parts designed to wear down over 5-10 years. When they're too short (~1cm or less), they lose firm contact with the commutator and the motor can't start.
All F21 causes:
1. Worn carbon brushes (40-50%) — the #1 cause.
2. Broken or slipped belt (15%) — motor spins but drum doesn't move, no load on motor = unusual behavior.
3. Failed tachogenerator (10%) — motor may actually be spinning but the sensor can't detect it.
4. Motor winding failure (10%) — open or short in motor windings.
5. Board motor relay failure (10%) — board can't deliver power to the motor.
6. Wiring/connector failure (10%) — broken wire or corroded connector between board and motor.
F-series codes on Bosch indicate the most critical system failures. While E-codes are often sensor or peripheral issues, F-codes mean a primary function completely failed.
How the board diagnoses F21:
1. Board applies voltage to the motor.
2. Waits for tachogenerator signal (any speed reading).
3. After ~10 seconds with zero signal → F21.
The most common cause by far: carbon brushes. In the millions of Bosch washers with brushed motors installed in homes worldwide, worn carbon brushes account for roughly 40-50% of all F21 cases. The brushes are consumable parts designed to wear down over 5-10 years. When they're too short (~1cm or less), they lose firm contact with the commutator and the motor can't start.
All F21 causes:
1. Worn carbon brushes (40-50%) — the #1 cause.
2. Broken or slipped belt (15%) — motor spins but drum doesn't move, no load on motor = unusual behavior.
3. Failed tachogenerator (10%) — motor may actually be spinning but the sensor can't detect it.
4. Motor winding failure (10%) — open or short in motor windings.
5. Board motor relay failure (10%) — board can't deliver power to the motor.
6. Wiring/connector failure (10%) — broken wire or corroded connector between board and motor.
What You're Probably Seeing Right Now
- Door locks, water fills, but the drum doesn't move.
- You may hear a brief hum or buzz as the motor tries to start, then silence.
- No sound at all from the motor — completely dead.
- F21 appeared gradually — the motor was slow or weak before failing completely.
- A burning or hot smell from the bottom of the machine — brushes sparking on the commutator.
DIY Fix — From Easiest to Hardest
1
Power Reset (2 minutes)
1. Unplug for 15 minutes.
2. Try a spin cycle.
3. **If motor starts:** Transient issue, but monitor closely.
4. **If F21 returns:** Physical diagnosis needed.
2. Try a spin cycle.
3. **If motor starts:** Transient issue, but monitor closely.
4. **If F21 returns:** Physical diagnosis needed.
2
Check the Belt (3 minutes)
1. Remove back panel.
2. Look at the belt between motor and drum pulleys.
3. **Belt on pulleys, tight?** Good — proceed to brushes.
4. **Belt off or snapped?** Replace. ~$15-25.
5. **Belt glazed (shiny)?** Slipping. Replace.
2. Look at the belt between motor and drum pulleys.
3. **Belt on pulleys, tight?** Good — proceed to brushes.
4. **Belt off or snapped?** Replace. ~$15-25.
5. **Belt glazed (shiny)?** Slipping. Replace.
3
Check and Replace Carbon Brushes (15 minutes)
**The most important F21 diagnosis:**
1. Locate motor (bottom of machine, behind back panel).
2. Find the two brush holders — one on each side of the motor.
3. **Unclip or unscrew** each brush holder and slide the brush out.
4. **Measure brush length:**
- New: ~3cm (30mm)
- Replace at: ~1cm (10mm)
- If shorter than 1cm → this is your problem
5. Replace both brushes as a pair.
6. After installing new brushes, **run 5-10 short cycles** with small loads to bed them in.
**Bedding in:** New brushes have a flat face that needs to wear into a curve matching the commutator. They'll spark initially — this is normal.
**Cost:** $10-25 per pair. This is one of the most satisfying DIY repairs — $15 in parts vs $200+ for a technician.
1. Locate motor (bottom of machine, behind back panel).
2. Find the two brush holders — one on each side of the motor.
3. **Unclip or unscrew** each brush holder and slide the brush out.
4. **Measure brush length:**
- New: ~3cm (30mm)
- Replace at: ~1cm (10mm)
- If shorter than 1cm → this is your problem
5. Replace both brushes as a pair.
6. After installing new brushes, **run 5-10 short cycles** with small loads to bed them in.
**Bedding in:** New brushes have a flat face that needs to wear into a curve matching the commutator. They'll spark initially — this is normal.
**Cost:** $10-25 per pair. This is one of the most satisfying DIY repairs — $15 in parts vs $200+ for a technician.
4
Check the Tachogenerator (5 minutes)
While the motor is accessible:
1. Find the tacho (small ring/disc on the shaft end opposite the belt).
2. Check it's **firmly attached.**
3. Measure resistance: **100-200Ω** expected.
4. OL or 0Ω = faulty.
**Loose tacho magnet** is surprisingly common — a drop of Loctite on the shaft fixes it.
1. Find the tacho (small ring/disc on the shaft end opposite the belt).
2. Check it's **firmly attached.**
3. Measure resistance: **100-200Ω** expected.
4. OL or 0Ω = faulty.
**Loose tacho magnet** is surprisingly common — a drop of Loctite on the shaft fixes it.
5
Check Motor Connections (5 minutes)
1. Find the large **motor connector** on the motor.
2. Disconnect, inspect pins for corrosion.
3. Reconnect firmly.
4. Trace wires toward the board — look for breaks.
**Vibration loosens connectors** over years. Reseating fixes ~10% of F21.
2. Disconnect, inspect pins for corrosion.
3. Reconnect firmly.
4. Trace wires toward the board — look for breaks.
**Vibration loosens connectors** over years. Reseating fixes ~10% of F21.
6
Test Motor Windings (5 minutes — Advanced)
1. Disconnect motor.
2. Resistance between motor terminals: **1-5Ω.**
3. Ground test: Each terminal to casing = **OL.**
4. If resistance is OL (open winding) — motor is dead.
5. If ground test shows connectivity — motor is grounding, dangerous.
**Dead motor:** Replacement ~$80-200 for the part.
2. Resistance between motor terminals: **1-5Ω.**
3. Ground test: Each terminal to casing = **OL.**
4. If resistance is OL (open winding) — motor is dead.
5. If ground test shows connectivity — motor is grounding, dangerous.
**Dead motor:** Replacement ~$80-200 for the part.
When to Call a Pro
- •Motor windings failed — motor replacement: $200-$400 installed.
- •Board motor relay dead — board repair: $150-$400.
- •Commutator severely scored — motor either needs re-machining or replacement: $150-$350.
- •EcoSilence motor failure — brushless motor and inverter: $250-$500.
What It'll Cost You
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