Saunier Duval F.85 Fault Code: Causes, Fixes & Repair Costs
What does the Saunier Duval F.85 fault code mean?
The F.85 code indicates that the boiler's PCB has detected a persistent, abnormal temperature difference between the flow and return NTC thermistors — the two sensors that monitor water temperature as it leaves and re-enters the boiler. This could mean the sensors are fitted to the wrong pipes, are reading incorrect resistance values, have wiring faults, or that heat transfer through the heat exchanger is poor. Because Saunier Duval is part of the Vaillant Group, the F.85 carries the same meaning across Saunier Duval and Vaillant boilers. The boiler will lock out and must be investigated by a Gas Safe registered engineer before normal operation can resume.
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
- Sensors fitted to the wrong pipes Common
The flow and return NTC thermistors must be fitted to their respective pipes — one where hot water leaves the boiler and one where cooled water returns. If they have been swapped or both clamped to the same pipe (often after a previous repair), the PCB will see a permanent, implausible temperature difference and trigger F.85.
- Out-of-calibration or poorly seated thermistors Common
NTC thermistors should produce a resistance of around 12 kΩ when measured at 20°C. If a sensor has drifted out of calibration, has corroded contacts, or is not making proper thermal contact with the pipe, its readings will be inaccurate and the PCB will flag the discrepancy.
- Damaged or loose wiring connections Sometimes
The wiring loom running from each thermistor back to the PCB can suffer from loose terminals, chafed insulation, or corrosion — particularly in older boilers or where condensation is present. A poor connection causes erratic or incorrect resistance signals that the PCB interprets as a sensor fault.
- Restricted heat exchanger or low gas rate Sometimes
If the heat exchanger is scaled up or partially blocked, or if the gas rate has been set incorrectly, heat transfer will be poor. This can create a permanently low differential between flow and return temperatures that triggers the F.85 lockout even when the sensors themselves are fine.
- PCB fault Rare
In a small number of cases the PCB itself misreads the resistance signals from otherwise healthy thermistors. This is usually only confirmed after the sensors and wiring have been tested and found to be in good order.
How to fix it
- Check the boiler display and note the fault code DIY safe
On most Saunier Duval models the display alternates between two numbers — the second has a decimal point after it — to indicate a fault. Confirm you are seeing F.85 so a correct diagnosis can be given when you contact an engineer.
- Attempt a single boiler reset DIY safe
Press and hold the reset button as described in your boiler's user guide. If the boiler restarts and runs normally, monitor it closely. If F.85 returns promptly or the boiler locks out again within a short time, do not keep resetting — repeated resets can mask a genuine fault and may cause further damage.
- Check that your gas supply is working DIY safe
Confirm other gas appliances in your home (e.g. a gas hob) are working normally. If there is no gas supply to any appliance, contact your gas supplier rather than an engineer.
- Do not attempt to access or move the temperature sensors Gas Safe engineer
The NTC thermistors are internal components attached to the boiler's pipework and wiring loom. Repositioning, disconnecting, or replacing them requires a Gas Safe registered engineer and should never be attempted by a homeowner.
- Do not interfere with the wiring or PCB Gas Safe engineer
Inspecting or repairing the wiring loom or PCB inside the boiler is not homeowner-safe work. Incorrect handling can create new faults, invalidate your warranty, and pose an electrical hazard.
- Have a Gas Safe engineer test the thermistors with a multimeter Gas Safe engineer
The engineer will disconnect each NTC thermistor and measure its resistance (expected ~12 kΩ at 20°C). A reading significantly above or below this indicates a faulty sensor. They will also check that each sensor is clamped to the correct pipe.
- Have the engineer inspect the wiring loom and connections Gas Safe engineer
All wiring between the thermistors and the PCB should be checked for continuity, correct terminal seating, and any signs of heat damage or corrosion. Faulty sections of wire or terminals will be repaired or replaced.
- Have the engineer assess heat exchanger performance and gas rate Gas Safe engineer
If sensors and wiring are healthy, the engineer should verify the gas input rate and check whether heat exchanger scaling or blockage is causing a persistently low temperature differential.
- Have the PCB tested if all other components are satisfactory Gas Safe engineer
Only once sensors, wiring, and heat exchanger performance have been ruled out should the PCB be investigated. A competent engineer will test the PCB's signal inputs and outputs rather than replacing it on assumption.
- Call a Gas Safe registered engineer Gas Safe engineer
F.85 is not a fault that can be resolved by homeowner action beyond a single reset attempt. Contact a Gas Safe registered engineer — check the register at gassaferegister.co.uk — and describe the F.85 code. Always ask for a written quote and confirm whether the call-out fee is included.
Parts you may need
- NTC flow thermistor · from £20
- NTC return thermistor · from £20
- Thermistor wiring harness/loom · from £35
- Main PCB · from £250
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 £150–£350, depending on the underlying cause.
Frequently asked questions
Can I fix the Saunier Duval F.85 fault myself?
No. While you can safely attempt one reset and check that your gas supply is on, the underlying cause of F.85 — whether it is a mispositioned sensor, a faulty thermistor, damaged wiring, or a PCB issue — requires a Gas Safe registered engineer to diagnose and repair. Attempting to access internal components yourself is unsafe and will likely void any remaining warranty.
How much does it cost to fix an F.85 fault on a Saunier Duval boiler?
Most homeowners pay between £150 and £350 to resolve an F.85 fault. Replacing one or both NTC thermistors, including parts and labour, typically falls in the £150–£200 range. If wiring repairs are also needed the cost rises toward the upper end of that range. In the less common scenario where the PCB needs replacing, costs can exceed £400 — but a good engineer will always rule out cheaper causes first before recommending a PCB replacement.
Why does the Saunier Duval F.85 code look the same as the Vaillant F.85?
Saunier Duval is owned by the Vaillant Group and shares the same underlying boiler technology and fault code architecture as Vaillant and Glow-worm. The F.85 code means the same thing across all three brands: a persistent, abnormal difference detected between the flow and return temperature sensor readings.
Will my boiler work at all when F.85 is showing?
No. F.85 is a lockout fault, which means the boiler shuts itself down completely and will not provide heating or hot water until the fault is cleared. This is a safety feature — running the boiler with unreliable temperature data could allow dangerously high temperatures to develop undetected. You may get a brief restart after pressing reset, but the boiler will lock out again if the root cause has not been fixed.