Saunier Duval F28 Fault Code: Ignition Failure at Start-Up
The F28 code means your Saunier Duval boiler has tried to light the burner three times in a row and failed every time. Rather than keep trying indefinitely — which could allow unburned gas to build up in the combustion chamber — the boiler locks itself out and displays F28 as a safety measure. The fault can stem from anything on the gas supply side, the ignition components, or the control electronics, and it affects all Saunier Duval models in the same way.
General guidance only — not a substitute for professional advice. 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.
What does the Saunier Duval F28 fault code mean?
The F28 code means your Saunier Duval boiler has tried to light the burner three times in a row and failed every time. Rather than keep trying indefinitely — which could allow unburned gas to build up in the combustion chamber — the boiler locks itself out and displays F28 as a safety measure. The fault can stem from anything on the gas supply side, the ignition components, or the control electronics, and it affects all Saunier Duval models in the same way.
Common causes
- No gas supply or very low gas pressure Common
If the gas meter is off, a supply interruption has occurred, or the dynamic pressure arriving at the boiler is below the minimum threshold, the burner simply cannot light. This is the first thing to rule out because it is easily checked without tools.
- Air in the gas line after a supply interruption Common
Even a brief, planned gas outage can allow air into the pipework. When the boiler restarts, the gas front has not yet reached the electrode by the time the three ignition attempts are made. The result is three clean misfires and a lockout — not a component failure as such, just an air pocket that needs purging.
- Dirty or damaged ionisation probe Common
Saunier Duval boilers confirm a flame is present by measuring a tiny rectification current (a few microamperes) through an ionisation probe. If the probe is coated in combustion deposits, misaligned, or cracked, it fails to report a flame even when one briefly exists. The control board sees no confirmation signal and cuts the gas as a precaution, locking out with F28.
- Faulty or worn ignition electrode or lead Common
The ignition electrode generates the spark that lights the gas. Over time the tip can erode, become carbon-fouled, or crack. The connecting lead and its connectors can also corrode or work loose. Any of these prevent a reliable spark from forming.
- Gas valve not opening Sometimes
If the gas valve solenoid is sticking, mechanically worn, or receiving incorrect drive signals, it will not open fully during the ignition sequence. The spark fires into an empty chamber and the boiler locks out.
- Seized or stiff water pump after summer inactivity Sometimes
After months of standing idle over summer, the circulator pump impeller can seize or drag. On some Saunier Duval models this interferes with the start-up sequence in a way that triggers F28 even though the ignition components themselves are fine.
- Faulty PCB (main control board) Rare
If the PCB is not sending the correct voltage at the right moment to the ignition transformer or gas valve, the entire ignition sequence breaks down. PCB faults are less common but should be investigated once all other components have been ruled out.
- Blocked flue or combustion air inlet Rare
A partially or fully obstructed flue terminal or air intake prevents the correct flow of combustion air, meaning the burner cannot sustain ignition. Nesting birds, debris, or a kinked flexible flue liner are typical causes.
How to fix it
- Check that your gas supply is working DIY safe
Try a gas hob ring or another gas appliance in the property. If nothing is working, there may be a wider supply issue — contact your gas supplier. Also visually check that the isolation valve on the boiler's gas inlet pipe is turned fully open (handle in line with the pipe).
- Reset the boiler — but no more than three times DIY safe
On most Saunier Duval models the display will flash two alternating numbers when a fault is present; the second number has a decimal point. Press and hold the Reset button until the boiler attempts to restart. Wait at least 30–60 seconds between each attempt to give any residual air in the gas line a chance to clear. Do not attempt more than two or three resets in total — repeated failed ignition attempts can overstress the ignition transformer.
- If a gas interruption recently occurred, allow a little extra time before resetting DIY safe
After a planned or unplanned gas outage, air will have entered the pipework. Waiting a few minutes before your single reset attempt gives the gas front more time to travel back to the boiler, improving the chance of a successful purge and light.
- Check the boiler's system pressure DIY safe
Look at the pressure gauge on the boiler — it should sit between 1 and 1.5 bar when the system is cold. If it reads below about 0.8 bar, top it up using the filling loop following your boiler's handbook. Very low pressure alone is unlikely to cause F28 but ruling it out takes only a moment.
- Inspect the flue terminal for obvious blockages DIY safe
Go outside and look at where the flue exits the building. Check for visible obstructions such as a bird nest, heavy debris, or ice in very cold weather. Do not attempt to dismantle any part of the flue — simply note what you can see and report it to the engineer.
- Call a Gas Safe registered engineer to diagnose and repair Gas Safe engineer
If the fault persists after the checks above, a registered engineer needs to attend. They will test gas inlet pressure with a manometer, inspect the ignition electrode and lead for wear or damage, clean or replace the ionisation probe, verify the gas valve is opening correctly, check the flue and air intake paths, and assess the PCB if all other components prove sound. Do not attempt to remove or adjust any gas or ignition components yourself.
Parts you may need
- Ignition electrode · from £25
- Ignition lead and connector · from £20
- Ionisation probe · from £30
- Ignition transformer · from £55
- Gas valve · from £150
- Circulator pump · from £90
- 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–£350, depending on the underlying cause.
Frequently asked questions
Can I fix the Saunier Duval F28 fault myself?
A few basic checks are safe for a homeowner: confirming the gas supply is on, attempting up to three resets, and topping up the system pressure if it is low. Beyond that, the cause almost certainly involves a gas or ignition component — the electrode, ionisation probe, gas valve, or PCB — and any work on these must be carried out by a Gas Safe registered engineer. Attempting it yourself is both illegal and dangerous.
My boiler showed F28 at the start of winter after sitting idle all summer — is it broken?
Not necessarily. Two things commonly happen after a long summer shutdown: air can work its way into the gas line, and the circulator pump impeller can stiffen or seize from lack of use. Try two or three resets with a minute's wait between each. If the boiler fires up, you are probably fine. If it still locks out, a Gas Safe engineer can check whether the pump needs freeing off or whether an ignition component has deteriorated over the summer.
How much does it cost to repair a Saunier Duval F28 fault in the UK?
Most F28 repairs fall in the range of £120–£350 once you include the call-out, labour, and any parts. A straightforward ionisation probe clean or electrode swap at the lower end; a gas valve replacement or pump change at the upper end. If the PCB turns out to be the culprit, parts alone can push the total higher — in that situation it is worth weighing the repair cost against the age of the boiler. As a rough rule of thumb, if the boiler is over 15 years old and the repair exceeds roughly half the cost of a new boiler, replacement often makes more financial sense.
Is it safe to keep resetting the boiler when F28 keeps coming back?
No — and there are two reasons to be cautious. First, repeated resets on an ignition fault mean gas is briefly released on every attempt; if the boiler is not lighting, that gas has to go somewhere. Make sure the area is ventilated, and if you smell gas at any point stop immediately, open windows, and call the National Gas Emergency Service on 0800 111 999. Second, each failed ignition attempt puts stress on the ignition transformer, and excessive resets can shorten its life. Limit yourself to two or three attempts, then call a Gas Safe engineer.