What this code means
The F42 code on a Bosch washer indicates that the control system has detected the motor spinning faster than expected during a cycle or self-test. This is an excessive drum speed fault. The control board monitors motor speed through a tachometer (tacho) feedback sensor mounted on the motor. When the feedback signal suggests the motor is running too fast, the washer stops and displays F42.
Bosch groups F42 with motor-related faults in service documentation. The code does not always mean the motor itself is bad. Instead, it often points to a problem in the speed-sensing circuit (tacho sensor, wiring, or connectors), worn carbon brushes on brushed motors, or faulty motor control electronics. Bosch advises power-cycling the machine once. If the code returns, qualified service is recommended.
Before You Replace Anything
Homeowners often replace the control board first when F42 appears. Check motor wiring, tacho connections, and carbon brushes before spending on a new board, since loose connectors or worn brushes cause the same symptom at a fraction of the cost.
Common Causes
- Faulty tachometer (tacho) sensor or circuit The tacho feedback sensor on the motor sends speed information to the control board, and a failing sensor or poor connection makes the board read incorrect speed and throw F42.
- Worn or damaged carbon brushes Brushed motors rely on carbon brushes to deliver current to the armature, and worn brushes cause erratic running, arcing, and speed-control problems that trigger overspeed faults.
- Loose or damaged motor wiring and connectors The harness between the motor, tacho, and control board can develop loose pins, corroded terminals, or burned contacts that interrupt feedback signals and create false overspeed readings.
- Failed drive motor Internal electrical or mechanical faults in the motor itself can cause uncontrolled speed or intermittent faults that the control system reads as overspeed.
- Defective control board or motor control electronics The control board’s motor-drive circuitry can fail and either command excessive speed or misinterpret correct feedback, triggering F42 even when the motor and tacho are good.
Step-by-Step Fix
- Unplug the washer from the wall outlet and turn off the water supply taps. F42 is a motor-control fault and you will be working near electrical connections.
- Access the motor compartment by removing the rear or bottom service panel (consult your model’s service manual for the exact location). You may need to tip the washer forward on a towel.
- Inspect all motor wiring and connectors for loose pins, burned contacts, or corrosion. Pay special attention to the tacho sensor connector (a small plug on the motor housing) and the main motor power connector. Wiggle each plug gently to check for looseness.
- Check the carbon brushes if your motor is a brushed type. Remove the brush caps or holders (usually two small covers on opposite sides of the motor) and slide out the brushes. Replace them if they are shorter than about 1 cm, chipped, or heavily arced.
- Test the tacho sensor connections by unplugging the tacho connector and checking for dirt, bent pins, or moisture. Reconnect firmly. If you have a multimeter and your model’s service data, measure tacho resistance (consult your model’s service sheet for the correct value).
- Clear the stored fault code by turning the program dial to off, then selecting the spin position. Hold the spin button down while moving the dial to the next position for about five seconds, then return to off. This resets the control memory on many Bosch and Siemens washers.
- Run a test cycle to see if F42 returns. If the code reappears immediately or within a few cycles, the motor, tacho sensor, or control board will need professional diagnosis and likely replacement.
Parts Often Needed
| Part | Notes |
|---|---|
| Bosch washer carbon brushes | Amazon | Specify your exact model number when ordering. Brushes are motor-specific and come in pairs with holders and springs. |
| Bosch washer tachometer (tacho) sensor | Amazon | Usually mounted on the motor housing. Confirm your motor type and part number from the service label inside the washer door or on the motor itself. |
| Bosch washer drive motor | Amazon | A complete motor assembly is needed if internal windings or rotor are faulty. Match the exact model and motor part number. |
| Bosch washer motor wiring harness | Amazon | Order if connectors are melted or wires are damaged beyond repair. Check the harness routing diagram in your service manual. |
When to Call a Pro
Call a qualified appliance technician if you are not comfortable working with electrical connections or if initial checks of connectors and brushes do not resolve F42. Diagnosing a tacho sensor or motor fault requires a multimeter and knowledge of the correct test points and values for your specific Bosch model. If the motor or control board needs replacement, the repair involves removing heavy components and working with live mains voltage during testing. Bosch recommends contacting their customer support at 1-800-944-2904 (U.S.) if the error persists after a power cycle. A professional service call typically costs between $150 and $350 depending on the part replaced and local labor rates.