Saunier Duval F.68 Fault Code: Causes, Fixes & Repair Costs
What does the Saunier Duval F.68 fault code mean?
The F.68 fault code appears when the boiler's PCB detects that the flame signal is erratic, too weak, or otherwise behaving unexpectedly during operation. In plain terms, the control board expects to see a consistent, stable flame once the burner has ignited — if the signal it receives from the ionisation circuit doesn't match that expectation, the boiler locks out and displays F.68 as a safety precaution. Because Saunier Duval is part of the Vaillant Group and both brands share the same control architecture, the F.68 code carries exactly the same definition across Saunier Duval and Vaillant boilers. The fault can be triggered by anything that disrupts combustion quality or corrupts the electrical signal feeding back to the PCB — from a low gas supply pressure through to a failing flame-sensing electrode or moisture on the circuit board.
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
- Low or fluctuating gas supply pressure Common
If the incoming gas pressure at the boiler is below the required level — or if it varies during a firing cycle due to air in the line or a supply issue — the burner flame will be weak and inconsistent. The PCB picks this up via the ionisation circuit and triggers F.68. This is one of the most frequently identified root causes.
- Faulty or incorrectly adjusted gas valve Common
The gas valve controls exactly how much gas enters the burner. If it has drifted out of calibration or has developed an internal fault, the air-to-gas ratio will be wrong, producing an unstable flame even when the gas supply pressure is fine. Old valves that cannot be recalibrated reliably typically need replacing rather than adjusting.
- Ionisation electrode or wiring fault Common
The ionisation electrode is the component that detects whether a flame is present by measuring a tiny electrical current through it. If the electrode is coated with deposits, cracked, or its cable is damaged or has a poor connection, the signal it sends to the PCB will be incorrect — triggering F.68 even if combustion itself is normal.
- Blocked or restricted condensate duct Sometimes
A partially blocked condensate outlet can cause a build-up of acidic water vapour inside the combustion chamber, which disrupts airflow and combustion quality. This in turn produces an erratic flame signal. A frozen external condensate pipe in cold weather is a common seasonal variant of this cause.
- Flue gas recirculation Sometimes
If the flue terminal is positioned where exhaust gases can be drawn back into the air intake — due to a nearby obstruction, an incorrectly installed flue, or strong winds — the combustion air becomes contaminated with CO₂. This degrades combustion quality and produces an unstable flame that the PCB flags as F.68.
- Moisture affecting the electronics Sometimes
Condensation or water ingress on the PCB or associated wiring can produce spurious signals in the flame-sensing circuit. This is more commonly seen on older Saunier Duval boilers — particularly those manufactured before around 2011 — where age and wear make moisture ingress more likely.
- Faulty PCB Rare
If the control board itself is generating or interpreting signals incorrectly, it may report an unstable flame even when the electrode, gas valve, and combustion are all functioning normally. A PCB fault is usually accompanied by other intermittent symptoms such as random cycling or unexplained lockouts, and it is typically investigated only after other components have been ruled out.
How to fix it
- Check your gas supply is working DIY safe
Before doing anything else, confirm that other gas appliances in the property — a hob or gas fire, for example — are working normally. If they are not, the problem lies with the gas supply rather than the boiler, and you should contact your gas supplier. If your supply is fine, move on to the next step.
- Inspect and thaw the condensate pipe if frozen DIY safe
In cold weather, check the condensate pipe — usually a white plastic pipe exiting through an external wall — for signs of freezing. If it feels solid or shows ice, you can carefully thaw it using warm (not boiling) water poured along the pipe. Once it's flowing freely, attempt a reset.
- Check and restore system pressure if low DIY safe
Look at the pressure gauge on the boiler's front panel. For most Saunier Duval boilers, normal operating pressure when cold is between 1.0 and 1.5 bar. If it reads below 1.0 bar, top it up using the filling loop — a braided hose with a valve, usually located beneath the boiler — until the gauge reads around 1.2–1.3 bar, then close the valve.
- Reset the boiler — but limit yourself to two or three attempts DIY safe
Press and hold the reset button (refer to your boiler's handbook for its exact location) until the display responds. Allow the boiler to run through its ignition sequence. If it fires successfully and runs normally, monitor it over the next day or two. If F.68 returns promptly, do not keep resetting — repeated resets without fixing the underlying cause can create additional problems and should be avoided.
- Have a Gas Safe engineer inspect the ionisation electrode and its wiring Gas Safe engineer
A registered engineer will remove and examine the flame-sensing electrode for cracking, carbon deposits, or corrosion, and check the condition and continuity of its connecting cable. A replacement electrode is a relatively inexpensive fix when this turns out to be the cause.
- Have the gas valve tested and adjusted or replaced Gas Safe engineer
The engineer will measure the gas inlet pressure and check the air-to-gas ratio across the burner using calibrated instruments. If the valve is out of adjustment, recalibration may resolve the fault. If the valve itself is faulty, replacement is usually the most practical outcome — old or worn valves rarely repay the cost of repair.
- Have the flue and air intake inspected for recirculation or blockage Gas Safe engineer
The engineer should check that the flue terminal is clear of debris, nesting materials, or nearby obstructions that could cause exhaust gases to recirculate into the air intake. The condensate trap and internal condensate routes should also be checked for blockage at this stage.
- Have the PCB inspected if all other components check out Gas Safe engineer
If gas pressure, the gas valve, the electrode, and the flue are all in good order, the engineer will turn attention to the PCB. A visual inspection looks for obvious damage such as burnt traces, corrosion, or cracked solder joints; a multimeter is then used to verify that the board is processing signals correctly. Replacement is the usual outcome if a fault is confirmed.
- Call a Gas Safe registered engineer Gas Safe engineer
If the boiler remains locked out on F.68 after you have checked the gas supply, thawed any frozen condensate, restored the system pressure, and attempted a reset, you need a Gas Safe registered engineer to carry out further diagnosis and any repairs. You can verify an engineer's registration at gassaferegister.co.uk.
Parts you may need
- Ionisation / flame-sensing electrode · from £25
- Electrode connecting cable and connector · from £18
- Gas valve (Saunier Duval compatible) · from £110
- Condensate trap assembly · from £22
- PCB (main control board) · from £220
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 F.68 myself without calling an engineer?
The only checks a homeowner should carry out are confirming the gas supply is on, thawing a frozen condensate pipe, topping up the system pressure via the filling loop, and attempting a reset no more than two or three times. Everything beyond that — testing the electrode, adjusting or replacing the gas valve, inspecting the flue, or working on the PCB — must be done by a Gas Safe registered engineer. Attempting gas or electronic repairs yourself is unsafe, illegal without the correct registration, and will typically void any remaining warranty on the boiler.
Why does F.68 keep coming back after I reset the boiler?
A reset clears the lockout but does nothing to address whatever is causing the unstable flame signal in the first place. If the fault returns quickly and repeatedly, the underlying cause — most often a gas pressure issue, a worn gas valve, or a failing ionisation electrode — is still present and getting worse. Repeatedly resetting the boiler in this situation is not recommended, as it can put additional strain on components and potentially mask a more serious safety issue. Book an engineer rather than continuing to reset.
How much does it typically cost to repair an F.68 fault on a Saunier Duval boiler?
For the most common repairs — replacing a flame-sensing electrode or having the gas valve recalibrated or replaced — most homeowners pay somewhere between £120 and £380 including parts and labour. A straightforward electrode replacement is usually at the lower end of that range, while a gas valve swap sits in the middle. If the PCB turns out to be the cause, a replacement board for a Saunier Duval model can add £200–£400 or more to the bill, though this is a less frequent outcome and worth discussing with your engineer before committing to the repair on an older boiler.
Is F.68 the same fault code on Vaillant boilers?
Yes. Saunier Duval is part of the Vaillant Group and the two brands share the same control electronics and fault-code system across their condensing boiler ranges. F.68 means exactly the same thing — an unstable or implausible flame signal — on both Vaillant and Saunier Duval boilers, and the diagnostic approach is identical.