Saunier Duval F.03 Fault Code: Causes, Fixes & Repair Costs
What does the Saunier Duval F.03 fault code mean?
The F.03 code on Saunier Duval boilers has two distinct meanings depending on which model series you own. On older Thema and ThemaClassic series boilers, F.03 signals a repetitive air or fan fault — the boiler previously detected an airflow problem (shown as F.02), attempted to recover automatically, and has now failed enough times that it has escalated into a hard lockout. On newer condensing models, F.03 can instead point to an interrupted or disconnected cylinder or heat-start temperature sensor (NTC), caused by a faulty sensor, damaged wiring, or a loose plug connection. Check your model's manual to confirm which applies to your boiler, but the diagnostic and safety steps below cover both scenarios.
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
- Blocked or obstructed flue Common
If the flue outlet outside is partially blocked by debris, birds' nesting material, overgrown foliage, or ice in cold weather, air cannot flow through the system correctly. This disrupts combustion airflow, triggers the initial F.02 air fault, and repeated failures escalate to F.03. This is one of the most straightforward causes to check from the outside before calling an engineer.
- Failing or seized fan Common
The fan draws combustion air through the boiler and expels flue gases. When bearings wear out, the fan may spin erratically, make unusual rumbling or grinding noises before the lockout, or stop altogether. Because the PCB monitors fan feedback to confirm it is running, a failing fan will repeatedly trigger the air fault condition that leads to F.03.
- Faulty or damaged NTC temperature sensor Common
On newer condensing Saunier Duval models, F.03 indicates the cylinder or heat-start NTC sensor circuit is open or interrupted. The sensor itself may have failed, the wiring could be damaged or corroded, or the plug connector may have worked loose. Without a valid temperature reading the boiler cannot operate safely.
- Defective airflow pressure switch or damaged membrane Sometimes
The pressure switch monitors the differential pressure created by the fan to confirm adequate airflow. If its rubber membrane cracks or perishes, or if the small plastic hoses connecting it become kinked or blocked, the switch cannot detect that the fan is running correctly — even when the fan itself is fine. This causes repeated false air fault signals.
- Loose or corroded fan wiring connections Sometimes
The PCB relies on electrical feedback from the fan to verify it is operating. Loose terminals, corroded connectors, or water-damaged wiring can break this signal, leading the boiler to believe the fan has failed when it may actually be running. Vibration over years of use can gradually loosen these connections.
- Leaking coaxial flue system Rare
On balanced-flue (room-sealed) boilers a leak in the coaxial flue assembly allows flue gases to recirculate into the air intake. This reduces the oxygen available for combustion and upsets the pressure differential the switch is monitoring, causing repeated air fault trips and eventually F.03 lockout.
- Faulty PCB Rare
If the control board itself has developed a fault it may misinterpret or fail to receive signals from the fan or NTC sensor, generating spurious fault codes. PCB failure is relatively uncommon compared with mechanical components but becomes more likely on boilers over ten years old or those that have suffered water ingress.
How to fix it
- Check your gas supply is on DIY safe
Before assuming a boiler fault, confirm the gas supply to the property is live. Check that other gas appliances (hob, gas fire) are working normally. If no gas appliances work, contact your gas supplier — there may be a supply interruption on your street.
- Inspect the flue outlet from outside DIY safe
Go outside and look at where the flue pipe exits the wall or roof. Make sure it is not visibly blocked by debris, a bird's nest, ice, or anything else. If it appears blocked by soft material you can safely remove, clear it, but do not attempt to dismantle any flue components.
- Reset the boiler once or twice DIY safe
Press and hold the reset button (on most models this is marked with a flame symbol or 'Reset') for around three seconds, then release. Allow the boiler a full minute to attempt a restart. If it fires up and runs normally, monitor it over the next day or two. Do not reset more than two or three times in succession — repeated resets without fixing the root cause will not resolve F.03 and may mask a worsening problem.
- Do not attempt to inspect or repair the fan, pressure switch, or any internal components Gas Safe engineer
Internal fan removal, pressure switch testing, and any work on gas-carrying or combustion components must only be carried out by a Gas Safe registered engineer. Attempting this yourself is unsafe, illegal for gas-related work, and will invalidate any remaining warranty.
- Do not attempt to replace or test the NTC sensor yourself Gas Safe engineer
If your model is showing F.03 as a sensor fault, the NTC and its associated wiring are inside the boiler. While the sensor itself is not a gas component, accessing and replacing it correctly requires knowledge of the boiler's wiring diagram and safe isolation procedures. Leave this to an engineer.
- Call a Gas Safe registered engineer to diagnose and repair Gas Safe engineer
An engineer will confirm whether your model is displaying F.03 as an air/fan fault or an NTC sensor fault, then systematically test the fan (including bearings and wiring), the airflow pressure switch and its connecting hoses, the flue integrity, and the NTC sensor circuit. They will replace whichever component is at fault and clear the lockout. Ask them to confirm your boiler is safe before they leave and request a copy of the service record.
Parts you may need
- Boiler fan / fan motor assembly · from £150
- Airflow pressure switch · from £45
- NTC temperature sensor · from £25
- Fan wiring harness / loom · from £35
- Printed circuit board (PCB) · from £180
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–£320, depending on the underlying cause.
Frequently asked questions
My Saunier Duval is showing F.03 — is it safe to keep using the boiler?
No. F.03 is a lockout code, meaning the boiler has shut itself down and will not restart until the fault is resolved. This is a safety feature — do not attempt to bypass it. The boiler should remain off until a Gas Safe registered engineer has inspected it and confirmed it is safe to operate.
What is the difference between F.02 and F.03 on a Saunier Duval boiler?
On Thema and ThemaClassic series boilers, F.02 is the initial air or fan fault code. The boiler will attempt to reset itself automatically after around 15 minutes. If that self-recovery fails repeatedly, the boiler escalates the fault to F.03, which is a hard lockout requiring a manual reset once the underlying problem has been fixed. Think of F.03 as the boiler saying it has already tried and failed to recover from F.02 several times.
How much does it cost to fix a Saunier Duval F.03 fault in the UK?
For most homeowners the total repair bill — parts and labour — falls between £120 and £320. A fan replacement typically costs £225–£275 all in, while a pressure switch or NTC sensor repair is usually cheaper at around £120–£180. PCB replacement is a less common outcome but can cost £300–£500 or more; at that price point, and if the boiler is over ten years old, many engineers would recommend considering a new boiler instead.
Can I reset a Saunier Duval F.03 fault myself?
You can press the reset button to attempt to clear the lockout, and it is worth trying this once or twice after checking the flue is unobstructed. However, F.03 is caused by a repeated underlying fault, so a reset alone will not fix the problem — the code is very likely to return within minutes or hours. A Gas Safe registered engineer needs to identify and repair the root cause before the boiler will stay running reliably.