F.025 High

Vaillant F.025 Fault Code

Also written as F025 — same fault.

The Vaillant F.025 fault code means: your boiler has detected that the exhaust gases are getting much hotter than they should be, or there is an issue with the electrical connection monitoring these temperatures.. It appears on 3 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: Wiring harness defective. The flue gas temperature is too high.
Typical repair cost £150-300 (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.025 fault code

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

Source

Vaillant F.025 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 7 July 2026