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Error code reference

Bosch Washer Error Code d:18

Peak voltage too high

Bosch codesBosch washer codes

Peak voltage too high — faulty motor control.

First step from the service manual

Run test P4; replace faulty motor control.

The complete diagnostic procedure includes additional test steps, resistance specifications, wiring diagram references, and component test points.

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What d:18 means on a Bosch washer

Error code d:18 on a Bosch washer indicates that the motor control unit has detected a peak voltage level that exceeds the acceptable threshold during operation. The motor control unit (MCU) continuously monitors electrical parameters in the drive circuit, including the voltage supplied to the drive motor. When this peak voltage rises beyond the safe operating range, the control system triggers d:18 to protect both the motor and the control electronics from damage caused by voltage spikes or sustained overvoltage conditions.

The most likely source of this fault is a failing or failed motor control board itself. The MCU contains power transistors, capacitors, and other components that regulate voltage delivery to the wash motor. If any of these components degrade or fail, the board may generate or fail to suppress internal voltage spikes, causing a false or legitimate overvoltage reading. External causes such as unstable household supply voltage or a failing power filter can also contribute to peak voltage events that the MCU registers.

Diagnosing this fault requires running the built-in test program P4, which exercises the motor control system and helps confirm whether the MCU is generating abnormal voltage readings under controlled conditions. If test P4 confirms the fault, replacement of the motor control unit is the indicated repair. Before replacing the MCU, technicians should also verify that the incoming line voltage is within specification and that the wiring harness connections to the motor control board are secure and free of corrosion or damage.

Source: manufacturer service documentation.

Common causes of d:18

  1. 01

    Faulty motor control board

    Part replacement

    The motor control unit itself is the most common cause of d:18. Internal component failure on the board can produce or fail to regulate voltage spikes that trigger this fault. Test P4 is used to confirm this diagnosis before replacement.

  2. 02

    Unstable or high incoming line voltage

    Wiring / connection

    A household power supply that delivers voltage spikes or sustained high voltage can push peak readings beyond the MCU's acceptable threshold. Check the supply voltage at the outlet with a multimeter to rule this out.

  3. 03

    Damaged wiring harness or connector to MCU

    Wiring / connection

    Loose, corroded, or damaged connector pins between the main control board and the motor control unit can cause erratic voltage readings. Inspect the harness connections at both ends for signs of heat damage, corrosion, or looseness.

  4. 04

    Failing drive motor

    Part replacement

    A motor with degraded windings or internal shorts can generate back-EMF voltage spikes that the MCU interprets as a peak overvoltage event. This is less common but worth evaluating if a new MCU does not resolve the fault.

See the test procedure for each cause

Frequently asked questions about d:18

What does d:18 mean on a Bosch washer?
Error code d:18 means the motor control unit has detected a peak voltage level that is too high during operation. This points to a fault in the motor control system. The washer stops operation to protect the motor and control electronics from damage caused by the overvoltage condition.
How do I fix error code d:18 on a Bosch washer?
The recommended diagnostic step is to run test program P4, which evaluates the motor control system under controlled conditions. If the test confirms the fault, the motor control unit needs to be replaced. Before replacing any parts, also verify that the household power supply voltage is stable and that all wiring harness connections to the motor control board are secure.
Will resetting my Bosch washer clear error code d:18?
A power reset may temporarily clear the code, but d:18 is a hardware-level fault tied to an actual overvoltage event in the motor control circuit. If the underlying cause, typically a faulty motor control board, is not repaired, the error will return as soon as the washer attempts to operate the motor again.
Is it safe to use my Bosch washer when it shows error code d:18?
You should not continue running the washer while d:18 is active. The error indicates a fault in the motor control system involving abnormal voltage levels, which can cause further damage to the motor or control electronics if the machine is forced to operate. Leave the washer powered off until the fault is diagnosed and repaired.
How much does it cost to repair a Bosch washer with error code d:18?
If the repair requires a motor control board replacement, parts and labor typically range from $150 to $400 depending on the specific model and local labor rates. If the fault is caused by a wiring or connection issue, the cost will be lower since no major parts are needed. Getting a diagnostic assessment from a qualified technician will give you a more precise estimate for your situation.

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