Vaillant F.86 fault codewhat it means and how to fix it
An ecoFIT code: the underfloor heating contact is open. A floor-protection thermostat wired to the boiler has tripped, or its sensor is disconnected, and heating is blocked until it closes.
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Get a Gas Safe registered engineer to look at your Vaillant F.86
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What F.86 means
F.86 appears only on the ecoFIT pure and ecoFIT sustain, whose boards carry a terminal for an underfloor heating safety contact. A wet underfloor system needs protecting from boiler flow temperatures that would damage the floor or the pipe, and the usual protection is a limit thermostat on the underfloor manifold wired back to the boiler so that, if the water going to the floor gets too hot, the contact opens and the boiler stops heating.
Vaillant print the row as "fault: underfloor heating contact — underfloor heating contact open, sensor disconnected or defective", and that is the whole of the diagnosis. Either the thermostat has tripped because the floor circuit really did overheat, or the wiring to it — or the sensor itself — has failed open, which the boiler cannot tell apart from a genuine trip.
The ecoFIT wiring diagram places the "safety thermostat for underfloor heating" on the Burner off terminal of the low-voltage plug X106, so an F.86 on a system with no underfloor heating points at that plug having been disturbed. The same boards also have a status form of the event, S.39 "underfloor heating contact open", which shows while the boiler waits for the contact to close; F.86 is the fault form, raised when the sensor is disconnected or defective. The ecoTEC plus, pro, exclusive, ecoTEC sustain and 2022 CS/1-5 tables do not carry the row.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Underfloor limit thermostat tripped | The floor circuit ran hotter than its thermostat allows, often because a mixing valve on the manifold has failed or been set too high. The contact opens and the boiler stops. | Common |
| Sensor or thermostat disconnected | Vaillant's "sensor disconnected". A wire off at the manifold or the boiler terminal reads exactly like a trip. | Common |
| Burner off terminal disturbed on a system with no underfloor heating | The contact lives on the low-voltage plug X106 alongside the room thermostat and eBUS terminals. Work on those connections can leave it open. | Occasional |
| Thermostat or sensor defective | Vaillant's last cause. The device has failed open and reports a trip regardless of temperature. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£160
Time on site
40 minutes
Restoring a bridge or a wire is a call-out, £80 to £120. A manifold limit thermostat is £100 to £160 fitted; a failed mixing valve is a heating engineer's job priced on the part.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Vaillant wiring harness (plug-and-cable set to a sensor or actuator)
£20–£65Authorised Vaillant parts distributor
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Vaillant fault codes
- F.49Get it checkedThe voltage on the eBUS, the two-wire link between the boiler and its Vaillant controls, has dropped too low. Usually a short or a wiring clash on that bus, not a boiler fault.Related: controls and communication
- conGet it checkedThe display has lost contact with the main board. "con" on a Vaillant is a communication fault between the user interface and the PCB in the electronics box, not a fault in the heating itself.Related: controls and communication
- S.39Get it checkedA protective wait, not a lockout: the "burner off" contact has opened — an underfloor thermostat or a condensate pump alarm wired to the boiler — and heating is blocked until it closes again.Related: controls and communication
- F.78Get it checkedA wiring-configuration code on UK combis: the external control's link box is connected but the hot water sensor input has not been bridged. Vaillant say the boiler itself has not malfunctioned.Related: controls and communication
- S.42Get it checkedA protective wait, not a lockout: an external device — a flue non-return flap through the VR40 module, or a condensate pump — is telling the boiler not to fire. Heating waits until it clears.Related: controls and communication
- F.90Get it checkedCommunication with the actoSTOR hot water module — or on the 2022 CS/1-5 the internal cylinder — has been interrupted. A wiring fault on the link, or a setting left on after the module went.Related: controls and communication
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Common questions about Vaillant F.86
I have no underfloor heating. Why F.86?
The boiler still has the terminal, on the same low-voltage plug as the room thermostat connections. If the code appeared after a control was wired in or the electronics box was opened, that plug has been disturbed; an engineer puts it right in minutes.
Is my floor damaged?
The thermostat exists to stop that. A trip means it did its job. If the trips repeat, the mixing valve on the underfloor manifold is the thing to have checked, since it is what sets the floor temperature.
Can I reset it?
A one-second press of the fault clearance key will clear it once the contact has closed again — that is, once the floor circuit has cooled or the wiring is restored. If the contact is still open the code stays.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.