Saunier Duval F.86 Fault Code: Causes, Fixes & Repair Costs
What does the Saunier Duval F.86 fault code mean?
The F.86 code appears on Saunier Duval boilers when the boiler's dedicated FLOOR safety input detects a problem with the underfloor heating safety thermostat circuit. This can mean one of two things: the underfloor heating circuit has reached an unsafe temperature and the safety thermostat has tripped to protect the system, or the FLOOR terminal on the boiler's PCB is reading an open circuit — which can happen if the wiring is loose, the thermostat itself has failed, or (very commonly in radiator-only homes) a shorting bridge that should be fitted across the FLOOR connector is missing or has fallen off. In short, the boiler cannot confirm the underfloor safety circuit is healthy, so it shuts down and locks out until the issue is resolved.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Missing or dislodged shorting bridge on the FLOOR connector Common
On boilers installed in homes with no underfloor heating, a small wire link (jumper or bridging plug) must be fitted across the FLOOR terminals on the PCB wiring centre to tell the boiler the circuit is complete. If this bridge was never installed, has worked loose over time, or was accidentally removed during a previous service, the boiler constantly sees an open circuit and triggers F.86 — even though there is nothing actually wrong with the heating system. This is the single most common cause of F.86 in standard radiator-only installations.
- Tripped underfloor heating safety thermostat Sometimes
Underfloor heating systems include a dedicated safety overheat thermostat designed to cut out if the floor temperature rises too high. If the flow temperature into the underfloor circuit has been set too high, or if a mixing valve or pump has malfunctioned, this thermostat can trip. Once tripped it breaks the FLOOR circuit, triggering F.86. Some safety thermostats reset automatically once the temperature drops; others require a manual reset button to be pressed.
- Loose or damaged wiring at the FLOOR terminal Sometimes
Vibration, age, or poor workmanship can cause the wiring connected to the boiler's FLOOR input to become loose or corroded. Even a partially disconnected wire is enough for the PCB to read an open circuit and raise the fault. Wiring to an external underfloor safety thermostat is particularly vulnerable as it runs outside the boiler casing.
- Faulty underfloor heating safety thermostat Sometimes
The safety thermostat itself can fail in an open-circuit state, making the boiler believe the floor has overheated even when it has not. This is more common on older thermostats or those that have previously tripped and been reset multiple times.
- PCB FLOOR input failure Rare
In a small number of cases the PCB's own FLOOR sensing input can develop a fault, causing it to misread a healthy circuit as open. This is relatively rare but should be considered once all wiring and thermostat checks have been completed and the fault persists.
How to fix it
- Check whether your home has underfloor heating DIY safe
Establish whether your property actually has an underfloor heating system connected to this boiler. If you have only radiators and no underfloor circuits, the most probable cause is a missing bridging plug on the FLOOR connector — tell your engineer this before they arrive, as it can save diagnostic time.
- Do not attempt to access internal wiring yourself Gas Safe engineer
The FLOOR connector and any bridging plug are located inside the boiler's wiring centre or directly on the PCB. Accessing these requires removing the boiler casing. Only a Gas Safe registered engineer should open the boiler and work on internal electrical connections.
- If you have underfloor heating, check the flow temperature setting DIY safe
Look at the boiler's control panel and any underfloor heating controller or manifold thermostat. If the flow temperature to the underfloor circuit is set higher than the system design temperature (typically 35–55 °C for underfloor), reducing it may prevent the safety thermostat from tripping again. This is a user-adjustable setting on most Saunier Duval models.
- Locate and reset the underfloor safety thermostat (if accessible and applicable) DIY safe
Some underfloor systems have a manual-reset safety thermostat mounted at the manifold or inside an airing cupboard — it looks like a small button on a round or rectangular component. If you can clearly identify this device and it has a visible reset button, pressing it is a homeowner-safe action. However, if the thermostat keeps tripping, do not keep resetting it — call an engineer to find out why it is overheating.
- Attempt a boiler reset DIY safe
Once you have made any safe adjustments (such as lowering the flow temperature or resetting an accessible safety thermostat), press and hold the boiler's reset button for approximately 3 seconds as described in your user manual. If the fault clears and the boiler fires up normally, monitor it over the next hour. If F.86 returns, do not reset more than 2–3 times in total.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
If the boiler locks out again or you cannot identify a clear reason for the fault, contact a Gas Safe registered engineer. They will check for a missing or dislodged bridging plug on the FLOOR connector, test continuity of the safety thermostat circuit, inspect all wiring to the FLOOR terminal, and assess the PCB input if wiring and thermostat are healthy. Always verify an engineer's Gas Safe registration at gassaferegister.co.uk before work begins.
Parts you may need
- Underfloor heating safety thermostat · from £35
- FLOOR connector bridging plug / jumper link · from £8
- PCB (printed circuit board) · from £280
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £80–£280, depending on the underlying cause.
Frequently asked questions
I don't have underfloor heating — why is my Saunier Duval showing F.86?
This is a well-known quirk of these boilers. When no underfloor heating is connected, the FLOOR terminals on the PCB must be bridged with a small wire link (shorting jumper) so the boiler registers a complete circuit. If that bridge is absent, loose, or was accidentally removed during a service, the boiler sees an open fault and displays F.86 even though nothing is actually overheating. A Gas Safe engineer can fit or reseat the jumper in a matter of minutes — it is usually one of the quickest and least expensive fixes for this code.
Is F.86 dangerous? Can I keep resetting the boiler?
F.86 is a safety lockout, not a gas or combustion fault, so it does not indicate an immediate risk of carbon monoxide or gas leak. However, you should not keep resetting the boiler indefinitely. If the underfloor circuit is genuinely overheating, repeatedly forcing the boiler back on could damage flooring, pipework, or other components. Limit yourself to 2–3 resets and call an engineer if the fault returns.
How much does it typically cost to fix F.86 on a Saunier Duval boiler?
For most households the repair falls between £80 and £280. A missing bridging plug is mostly a labour charge — expect £80–£150 for a call-out and the fix. Replacing a faulty underfloor safety thermostat typically costs £100–£250 including parts and labour. If the PCB FLOOR input has failed, a new PCB can add significantly to the bill — often £250–£450 or more — but this is a relatively rare outcome and worth discussing with your engineer before committing to the repair on an older boiler.
What should I tell the engineer before they arrive?
Let them know whether you have an underfloor heating system or only radiators, what flow temperature the boiler is set to, whether F.86 appeared suddenly or came on gradually, and how many times you have reset the boiler. If you have no underfloor heating, specifically ask them to check the FLOOR connector bridging plug first — it is the most common cause and the quickest thing to rule out.