F.25 High

Vaillant F.25 Fault Code

Also written as F25 — same fault.

The Vaillant F.25 fault code means: your boiler has detected an electrical connection issue or a sudden temperature rise, causing it to shut down for safety.. It appears on 45 Vaillant models in our database.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description: Interruption in the compact thermal module cable harness

What causes the Vaillant F.25 fault?

This fault occurs when the boiler's safety sensors detect that the internal temperature has risen too quickly or that the electrical connection to the thermal module has been broken. The most common reason is a loose or damaged wiring harness, but it can also be triggered if the heat exchanger becomes too hot due to a lack of water flow. To prevent damage, the system automatically shuts down until the electrical circuit is restored or the unit cools down.

Typical repair cost £120-350 (including labour)

Engineer required

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

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Vaillant models that show the F.25 fault code

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

Source

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

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