BoilerFaultCodesVaillant 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 26 Vaillant models in our database.
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.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
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A cover plan would handle faults like F.86: no call-out fees, no surprise bills.
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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.
- ecoFIT Pure 435 Repair: £120-250
- ecoFIT pure 612
- ecoFIT pure 615
- ecoFIT pure 618
- ecoFIT pure 630
- ecoFIT Pure 825 Repair: £120-250
- ecoFIT Pure 830 Repair: £120-250
- ecoFIT Pure 835 Repair: £120-250
- ecoFIT sustain 615
- ecoFIT sustain 618
- ecoFIT sustain 630
- ecoFIT sustain 825
- ecoFIT sustain 830
- ecoFIT sustain 835
- ecoTEC plus 412 Repair: £120-250
- ecoTEC plus 415 Repair: £120-250
- ecoTEC plus 418 Repair: £120-250
- ecoTEC plus 424 Repair: £120-250
- ecoTEC plus 430 Repair: £120-250
- ecoTEC Plus 610 Repair: £120-250
- ecoTEC Plus 620 Repair: £90-180
- ecoTEC Plus 625 Repair: £90-180
- ecoTEC Plus 635 Repair: £90-180
- ecoTEC Plus 840 Repair: £90-180
- Home Combi Repair: £120-250
- Home System Repair: £90-180
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.
- Vaillant ecoFIT Pure 835 manual (PDF)
- Vaillant ecoFIT pure 630 manual (PDF)
- Vaillant ecoFIT sustain 630 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026