F.86 High

Vaillant F.86 Fault Code

Also written as F86 — same fault.

The Vaillant F.86 fault code means: your boiler has stopped providing heat because a safety limit switch for your underfloor heating has been triggered or disconnected.. It appears on 24 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: Fault: Underfloor heating contact. Underfloor heating contact open, sensor disconnected or defective

What causes the Vaillant F.86 fault?

This fault occurs because a safety switch designed to protect your underfloor heating has detected that the water temperature is too high, causing it to cut power to the boiler. The most common reasons are a failed pump that isn't moving heat away quickly enough or a fault with the limit switch itself. It acts as a safety barrier to prevent the high heat from damaging your floor surfaces.

This fault is more common during the autumn and winter months when the underfloor heating is first switched on after being dormant all summer.

Typical repair cost £90-180 (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 27 April 2026