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Error code reference

Bosch Washer Error Code 25

Error of Aqua Sensor (Turbidity Sensor)

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Error of Aqua Sensor (Turbidity Sensor) - calibration of operating point failed, calcified sensor.

First step from the service manual

Check Aqua Sensor, check wire harness.

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

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What 25 means on a Bosch washer

Error code 25 on a Bosch washer indicates a failure in the Aqua Sensor system, specifically that the sensor's calibration of its operating point has failed. The Aqua Sensor is a turbidity sensor that monitors water clarity during the wash and rinse cycles. It measures how much light passes through the water to determine soil levels and adjust cycle parameters accordingly. When the control board attempts to calibrate the sensor's baseline reading and cannot establish a valid reference point, error code 25 is triggered.

The most common root cause of this error is calcium and mineral scale buildup on the sensor's optical components. In hard water areas, limescale deposits accumulate on the sensor lens or window over time, blocking or scattering the light beam used for turbidity measurement. This prevents the sensor from achieving a clean baseline reading during calibration, causing the fault to trigger. The sensor is typically located in the sump area or along the circulation path where it can sample wash water continuously.

A secondary cause is a wiring or connection issue between the Aqua Sensor and the main control board. A damaged wire harness, corroded connector terminals, or a loose plug can interrupt the signal and produce the same calibration failure. Diagnosing this code requires physically inspecting the sensor for visible scale deposits or damage, then checking continuity and secure connection of the wire harness before concluding the sensor itself requires replacement.

Source: manufacturer service documentation.

Common causes of 25

  1. 01

    Calcified or fouled Aqua Sensor

    Wiring / connection

    Limescale and mineral deposits from hard water coat the optical surface of the turbidity sensor, preventing it from calibrating a clean baseline reading. The sensor lens may appear cloudy or have a white crusty residue when inspected.

  2. 02

    Faulty Aqua Sensor

    Part replacement

    The sensor's internal optical or electronic components may have failed outright, making calibration impossible even after cleaning. If descaling the sensor does not clear the fault, sensor replacement is required.

  3. 03

    Damaged or loose wire harness

    Wiring / connection

    A corroded connector, pinched wire, or loose plug between the Aqua Sensor and the control board can interrupt the signal and cause a calibration failure. Inspect the harness for visible damage and confirm all connectors are fully seated.

  4. 04

    Sensor installed incorrectly or dislodged

    Wiring / connection

    If the sensor is not properly seated in its mounting position, it may not be able to sample water correctly, leading to failed calibration. Check that the sensor is correctly positioned and fully locked in place.

  5. 05

    Control board communication fault

    Part replacement

    In rare cases, the main control board may not be sending or receiving the calibration signal correctly, triggering error 25 even when the sensor and wiring are intact. This is typically diagnosed after all other causes have been ruled out.

See the test procedure for each cause

Frequently asked questions about 25

What does 25 mean on a Bosch washer?
Error code 25 on a Bosch washer indicates that the Aqua Sensor, also called the turbidity sensor, has failed to calibrate its operating point. This sensor monitors water clarity to optimize wash and rinse cycles. The most common cause is limescale buildup on the sensor from hard water, though wiring issues or a failed sensor can also trigger this code.
Can I fix error code 25 on my Bosch washer myself?
In some cases, yes. If the Aqua Sensor is coated in limescale, cleaning the sensor with a descaling solution may restore normal function. You should also check that the wire harness connected to the sensor is secure and undamaged. If cleaning does not resolve the fault, the sensor likely needs to be replaced, which requires some disassembly and comfort working with appliance wiring.
Will resetting my Bosch washer clear error code 25?
A reset may temporarily clear the error code from the display, but it will return if the underlying cause is not addressed. The sensor calibration failure is tied to a physical condition, such as scale buildup or a hardware fault, that a reset cannot fix. The sensor or wiring issue must be resolved for the code to clear permanently.
Is it safe to use my Bosch washer when error code 25 is showing?
Error code 25 affects the washer's ability to monitor water turbidity and optimize its cycles, but it does not represent an immediate safety hazard. However, running the machine in a fault state can lead to inefficient washing or improper rinsing, and continuing to ignore the fault may stress other components over time. It is best to diagnose and repair the issue before regular use continues.
How much does it cost to repair error code 25 on a Bosch washer?
If the repair is just a cleaning of the Aqua Sensor, costs are minimal, mainly your time or a small service call fee. If the turbidity sensor needs to be replaced, parts typically range from around 30 to 80 dollars depending on the model, with professional labor adding 80 to 150 dollars or more depending on your area. A full diagnosis by a technician will give you a more accurate estimate for your specific situation.

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