How much does it cost to fix Viessmann error code 25:1?

Typical repair cost for the Viessmann 25:1 fault is £120-200 including labour.

25:1 Low

Viessmann 25:1 Fault Code

Q: What does the Viessmann 25:1 fault code mean?

Your boiler has lost the connection to the thermometer placed outside your home, so it cannot automatically adjust your heating based on the weather.

Also written as 251 — same fault.

The Viessmann 25:1 fault code means: your boiler has lost the connection to the thermometer placed outside your home, so it cannot automatically adjust your heating based on the weather.. It appears on 6 Viessmann models in our database.

Technical description: Recognition of outside temperature sensor and fault monitoring

Typical repair cost £120-200 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Viessmann models that show the 25:1 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Viessmann 25:1 fault code information on this page is sourced and cross-checked against the official Viessmann service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026