F.86 Medium

Vaillant F.86 Fault Code

Also written as F86 — same fault.

The Vaillant F.86 fault code means: your boiler has detected that the external safety switch for your underfloor heating system is reporting a fault, so it has shut down to prevent potential overheating or damage.. It appears on 9 Vaillant models in our database.

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description: Fault: Underfloor heating contact
Typical repair cost £120-250 (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.86 fault code

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

Source

Vaillant F.86 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