Saunier Duval F.27 Fault Code: Causes, Fixes & Repair Costs
What does the Saunier Duval F.27 fault code mean?
The F.27 fault code on Saunier Duval Condens boilers signals one of two closely related problems, depending on the exact model variant. On some Condens models it indicates a vortex sensor fault — the sensor that monitors gas-air mixing in the venturi tube is reporting an anomaly. On others, it means the boiler's control board has detected a flame signal at a point in the firing sequence when no flame should be present, commonly called false flame detection or flame simulation. Either way, the boiler locks out as a safety precaution, leaving you without heating or hot water until the underlying cause is diagnosed and resolved. Because both issues relate directly to gas combustion and flame control, F.27 should always be taken seriously — particularly if you notice any smell of gas near the boiler or flue.
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
- Moisture or water ingress on the PCB Common
If water is leaking inside the boiler casing — from a worn seal, a failing diverter valve, or a loose internal pipe — it can settle on the circuit board. Even a small amount of moisture can bridge connections on the PCB, creating a spurious flame signal that the boiler's logic interprets as a real flame, triggering the F.27 lockout.
- Worn or faulty ignition electrode and lead Common
The ignition electrode and its high-voltage lead serve a dual purpose: they create the spark to light the gas and then sense the resulting flame via flame rectification. When these components corrode, crack, or develop an internal short, they can send erratic signals back to the PCB — in some cases producing a signal that mimics a detected flame, causing the false-flame lockout even before ignition is attempted.
- Degraded or failed PCB Sometimes
Over years of operation, electronic components on the printed circuit board naturally age. If the PCB develops a fault — without any water damage — it may misread or manufacture signals from the flame-sensing circuit, resulting in repeated F.27 lockouts even when all other components are in good condition.
- Faulty or passing gas valve Sometimes
A gas valve that is not sealing fully when commanded to close can allow a small trickle of gas through, potentially sustaining a flicker of flame when the boiler should be off. The PCB then correctly detects this rogue flame and locks the boiler out on F.27. This is the least common cause but carries the most immediate safety risk — especially if accompanied by a smell of gas.
- Water ingress via the flue Rare
On older installations with a vertical open flue, rainwater can travel down the flue and into the gas valve solenoid or onto nearby electrical components, producing intermittent false-flame signals. This tends to occur seasonally and may correlate with heavy rainfall.
How to fix it
- Check for a gas smell and ventilate if necessary DIY safe
Before doing anything else, check whether you can smell gas near the boiler or flue terminal outside. If you can, do not touch any electrical switches or the boiler controls. Open windows and doors to ventilate, turn off the gas at the meter if safe to reach, leave the property, and call the National Gas Emergency Service on 0800 111 999 from outside. Only return when a Gas Safe engineer has confirmed it is safe. If there is no gas smell, proceed to the next step.
- Attempt a single boiler reset DIY safe
Press and hold the reset button on your Saunier Duval boiler for approximately three seconds, then release. If F.27 appeared following a power cut or a brief electrical disturbance, one reset may be enough to clear it. Watch the display — if the fault returns immediately or within a few minutes of the boiler attempting to fire, do not keep resetting. Repeated resets on a genuine F.27 fault will not fix the underlying problem and could mask a safety issue.
- Inspect around the boiler for visible water leaks DIY safe
With the boiler switched off at the programmer or room thermostat (not at the mains), look carefully around the base of the boiler and along visible pipework for any drips, staining, or damp patches. If you spot a leak, note its approximate location — this information will help the engineer. Do not attempt to open the boiler casing or touch any internal components yourself.
- Have a Gas Safe engineer inspect the ignition electrode and lead Gas Safe engineer
An engineer will remove the casing and use a multimeter to check the resistance and continuity of the ignition electrode, its lead, and the associated wiring harness. A cracked electrode tip, a degraded high-voltage lead, or a loose wiring connection can all produce the false-flame signal that causes F.27. Cleaning or replacing these parts is relatively straightforward and is often the most cost-effective fix.
- Have the PCB tested for moisture damage and electrical faults Gas Safe engineer
If the ignition components test correctly, the engineer will examine the PCB for signs of corrosion, burn marks, or moisture ingress. If water damage is found, the source of the leak must be repaired before a new board is fitted — installing a replacement PCB into a still-leaking boiler would only repeat the problem. PCB replacement requires Gas Safe competency and must not be attempted by a homeowner.
- Have the gas valve tested and replaced if necessary Gas Safe engineer
If both the ignition components and PCB are found to be in good order, the engineer will test the gas valve to confirm it is sealing correctly when closed. A 'passing' valve that allows gas to trickle through is a safety concern and must be replaced by a Gas Safe registered engineer. Gas valve work is a legally restricted activity in the UK.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
F.27 involves gas combustion components and high-voltage electronics. Beyond the basic checks above, all diagnostic work and parts replacement must be carried out by a Gas Safe registered engineer. You can find one via the Gas Safe Register website (gassaferegister.co.uk) or by contacting Saunier Duval's service line for an approved engineer in your area.
Parts you may need
- Ignition electrode · from £35
- Ignition lead / HT lead · from £25
- PCB (printed circuit board) · from £220
- Gas valve · from £180
- Wiring harness · from £55
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 £120–£380, depending on the underlying cause.
Frequently asked questions
Can I fix the Saunier Duval F.27 fault myself?
No. Beyond attempting a single reset and checking for obvious leaks or a gas smell, F.27 requires a Gas Safe registered engineer. The fault involves either the gas valve, ignition components, or the PCB — all of which are legally restricted from DIY work in the UK. Attempting your own repair could void your warranty, invalidate your home insurance, and put your household at risk.
My F.27 keeps coming back after I reset it — what does that mean?
A fault that returns immediately or within a short time of resetting almost always indicates a genuine component failure rather than a one-off electrical glitch. The most common culprits are a degraded ignition electrode or lead, moisture on the PCB, or — less commonly — a passing gas valve. Continuing to reset the boiler without addressing the root cause is not recommended, as it will not resolve the fault and could delay identifying a safety issue.
How much does it cost to repair an F.27 fault in the UK?
Most homeowners pay between £120 and £380 all-in (parts and labour) to resolve an F.27 fault. Replacing the ignition electrode and lead is at the lower end; PCB replacement sits in the middle. If the gas valve also needs replacing, costs can push higher. In rare cases where both the PCB and gas valve have failed simultaneously, repair bills can exceed £500 — at that point, it is worth getting a new-boiler quote for comparison, particularly if your Saunier Duval is over ten years old.
Is F.27 the same as a gas leak on my Saunier Duval?
Not necessarily, but treat it seriously. F.27 most commonly points to an electrical fault that produces a false flame signal — there may be no actual gas leak involved. However, if a passing gas valve is the cause, a small amount of gas could be escaping into the combustion chamber when the boiler is off, which is a safety concern. If you smell gas at any point, follow the emergency steps: ventilate, shut off the gas supply, leave the property, and call 0800 111 999 immediately.