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Vaillant F35 Fault Code: What It Means and What To Do
An F35 on a Vaillant boiler points to an air or flue gas duct fault — the boiler has detected a problem with how combustion gases are being expelled or fresh air drawn in. Because it involves the flue and sealed combustion circuit, it is an engineer-only fault.
Quick answer
F35 means an air/flue gas duct fault. Your Vaillant has sensed that the flue is blocked, restricted, recirculating gases or that the fan/air pressure system is not performing correctly. The boiler shuts down deliberately to keep you safe.
The only thing a homeowner should do is step outside and check the external flue terminal is not blocked by leaves, a bird's nest, snow or debris. You may also try a single reset. If F35 returns, stop and call a Gas Safe registered engineer — never open the boiler or touch the flue yourself.
What does F35 mean on a Vaillant boiler?
The F35 fault code on a Vaillant boiler indicates an air or flue gas duct fault. In plain terms, the boiler's control board has decided that combustion gases are not leaving the appliance safely, or that fresh air is not being drawn in correctly.
This can be caused by a physical blockage, a leak in the flue, recirculation of flue gases back into the air intake, or a problem with the fan and the air pressure sensing system.
Vaillant fits this protection across many of its popular ranges, including the ecoTEC plus, ecoTEC pro and ecoFIT pure combi and system boilers.
The exact wording can vary slightly between models, but the principle is the same: F35 is a flue and combustion safety fault, and the boiler locks out rather than continue running in a potentially unsafe state.
That lockout is the boiler doing its job — it is far safer to have no heating for a day than to run a boiler that cannot vent properly.
F35 is a flue-safety lockout, not a nuisance code. A blocked or leaking flue can allow carbon monoxide to escape. Do not bypass, ignore or repeatedly reset an F35 to force the boiler back on — beyond one reset and the external flue check, it must be inspected by a Gas Safe registered engineer.
Common causes of F35
- A blocked or restricted external flue terminal — leaves, a bird's nest, cobwebs, snow or general debris covering the flue outlet outside.
- Flue gas recirculation — exhaust gases being drawn back into the air intake, often due to a damaged, poorly fitted or incorrectly sealed flue.
- A leaking or disconnected flue joint inside the property, in a void or in the loft.
- A faulty or failing fan, which moves air through the combustion chamber and flue.
- A faulty air pressure switch or air pressure sensor (APS) giving the board a false reading.
- A blocked condensate pipe in some cases, or a wiring/cable harness fault between the fan, sensor and PCB.
- A defective printed circuit board (PCB) misreading the combustion conditions.
Most of these sit inside the sealed combustion circuit, which is exactly why F35 is treated as engineer-only. The one cause you can rule out yourself is an obviously blocked outdoor flue terminal. If you suspect a frozen condensate pipe is involved, our guide on a frozen condensate pipe explains the safe thaw steps.
What you can safely check
F35 is an engineer-only fault, so there is no safe DIY repair. There are, however, two universal, no-risk things a homeowner can do before booking an engineer:
- Look at the external flue terminal from outside. This is the plastic or metal pipe poking out of your wall or roof. Clear away any obvious leaves, snow, nests or debris around the opening. Do not poke anything inside it, and do not stand on anything unsafe to reach it.
- Try one reset. Press and hold the reset button on the boiler's display for a few seconds. If you are unsure how, see our guide on how to reset your boiler. Reset the boiler once only — if F35 comes straight back, leave it off and book an engineer.
That is the full extent of safe homeowner action for F35. You must not remove the boiler's front casing, dismantle the flue, touch the fan, or interfere with any gas, combustion or sealed component. Doing so is dangerous and, for gas work, illegal. For more on what a lockout means and why it should not be overridden, see our guide to a boiler lockout.
When to call a Gas Safe registered engineer
If the external terminal is clear and a single reset does not hold, the fault is internal and needs a professional. Only a Gas Safe Register engineer may work on the flue, fan, air pressure sensor, gas supply, burner or PCB. It is illegal and unsafe for anyone not on the register to attempt this work.
What the engineer will check
- Inspect the full flue run for blockages, leaks, slope, seals and correct installation.
- Test the fan output and the air pressure switch/sensor readings.
- Check for flue gas recirculation and analyse combustion with a flue gas analyser.
- Inspect the wiring harness and connections to the fan, sensor and PCB.
- Confirm the condensate route is clear and the PCB is reading correctly.
If you ever smell gas or suspect carbon monoxide (headaches, dizziness, a yellow rather than blue flame), leave the property and call the National Gas Emergency line on 0800 111 999.
Typical Vaillant F35 repair cost
Cost depends entirely on what the engineer finds. Clearing a blocked terminal can be inexpensive; replacing a fan or PCB is the costlier end. These are indicative 2026 UK ranges including parts and labour — get a firm quote from your engineer.
| Job | Indicative 2026 cost |
|---|---|
| Diagnostic / call-out | £70 – £120 |
| Clear blocked flue terminal | £80 – £150 |
| Flue repair / reseal section | £150 – £350 |
| Fan replacement | £185 – £350 |
| Air pressure switch / sensor | £120 – £220 |
| PCB replacement | £300 – £450 |
For wider context on how these compare to other faults, see our boiler repair costs guide.
Related Vaillant codes
If you are working through a string of Vaillant faults, these sibling guides may help:
- Vaillant F75 — pump/pressure sensor fault.
- Vaillant F28 — ignition failure.
- Vaillant F22 — low water pressure / dry fire.
For the full list, see our Vaillant fault codes hub.
Will boiler cover pay for an F35 repair?
It depends on your policy.
Faults involving the fan, flue, air pressure sensor and PCB are the kind of breakdown many boiler cover policies are designed to handle, but cover is never guaranteed — every policy has its own terms. Common conditions and exclusions to look out for include an excess (often £60–£99), an age limit on older boilers, a brief no-claims period at the start of a new policy, a requirement that the boiler was serviced and in working order when you took the policy out, and the fact that pre-existing faults are not covered.
Always check your own policy wording, as only your insurer can confirm whether a specific repair is covered.
The following is general information, not financial or insurance advice. If you want to understand what is and isn't typically included before you buy, read what boiler cover includes and our honest take on whether boiler cover is worth it. For a one-off F35 with no cover, you are looking at a single engineer visit at the costs above.
Is a Vaillant F35 fault dangerous?
It can be, which is why the boiler shuts itself down. F35 signals an air or flue gas duct fault, and a blocked or leaking flue could allow carbon monoxide to escape. The lockout is a safety feature. Do not force the boiler back on beyond one reset, and have it checked by a Gas Safe engineer. If you smell gas or feel unwell, leave and call 0800 111 999.
Can I fix F35 myself?
No — there is no safe DIY repair for F35. The only things you should do are check the external flue terminal outside for obvious blockages (leaves, snow, a nest) and try one reset. Everything else involves the flue, fan or sealed combustion circuit and is engineer-only. It is illegal for anyone not Gas Safe registered to work on these parts.
Will F35 clear on a reset?
Sometimes a single reset clears it if the cause was momentary — for example wind briefly disturbing the flue. But if F35 returns straight after a reset, the fault is real and internal. Reset once only; repeatedly resetting a flue-safety lockout is unsafe. Book an engineer if it comes back.
How much does it cost to repair a Vaillant F35?
It depends on the cause. A diagnostic call-out is typically £70–£120 in 2026. Clearing a blocked terminal is cheap; a fan is around £185–£350, an air pressure sensor £120–£220, and a PCB £300–£450, parts and labour included. Always get a firm quote from your engineer.
Why does my boiler keep showing F35 after I cleared the flue outside?
If the external terminal is clear but F35 persists, the problem is internal — a leaking flue joint, recirculation, a failing fan, a faulty air pressure sensor or a PCB issue. These are inside the sealed parts of the boiler and need a Gas Safe engineer to diagnose with the casing removed and a flue gas analyser.
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Compare boiler coverThis article is general information, not gas-safety or financial advice. Always have gas appliances checked and repaired by a Gas Safe registered engineer. In a gas emergency, call the National Gas Emergency line on 0800 111 999. Costs are indicative UK guides for 2026.