Vaillant F.334 fault codewhat it means and how to fix it
A commissioning code on the 2022 ecoTEC plus: the flue system configuration was never completed in the installer settings. F.336, F.337 and F.338 are its neighbours — a configuration set wrongly.
Take care
Get a Gas Safe registered engineer to look at your Vaillant F.334
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What F.334 means
The ecoTEC plus CS/1-5 has to be told what flue it is connected to. Its combustion control adjusts for flue length and type, and until the installer has entered the flue configuration the boiler will not run: F.334 is "the configuration of the flue system was not completed — check the flue gas configuration and complete this".
Three related rows sit beside it. F.336 is "the unit is not permitted in this configuration — flue system configuration set incorrectly: set the approved flue system configuration; incorrect gas type set: check the gas type and the gas type setting under D.156". F.337 is "the multiple-flue configuration conversion set is not suitable for the heat cell", for a shared-flue installation whose conversion kit — an air intake pipe with an integrated non-return valve and air-mass flow sensor — does not match the boiler. F.338 is "incorrect multiple-flue configuration setting — set the approved flue system configuration". None of these is a fault in the sense of something having broken.
They are the boiler refusing to fire until its settings describe the installation it is in, and every one of them lives in the code-protected installer level, which the operating instructions do not give to the householder. They appear at commissioning, after a board or display change that has wiped the settings, or after a flue alteration that was not entered. The right response is to ring the installer, because the configuration is theirs to complete and, when the boiler is new, it is part of the installation they were paid for.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Flue configuration never entered at commissioning | F.334's only cause. The installer did not complete the flue system setting, and the boiler has waited for it. | Common |
| Settings lost after a board or display change | A replacement PCB or display starts with the configuration incomplete; the engineer fitting it has to re-enter the flue system as well as the gas type. | Common |
| Configuration set to one the boiler does not permit | F.336 and F.338. A flue type or a multiple-flue setting the boiler is not approved for, or a gas type under D.156 that does not match. | Occasional |
| Wrong multiple-flue conversion set | F.337. A shared-flue kit that does not suit the heat cell; Vaillant's remedy is to replace the conversion set. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£120
Time on site
30 minutes
Completing the configuration is a short visit, £80 to £120, and should be free when it follows the firm's own installation or repair. No parts are involved unless F.337 calls for a different conversion set.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Vaillant main PCB (electronics box)
£160–£340Authorised Vaillant parts distributor
Price checked 28 September 2026
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Related Vaillant fault codes
- F.32Get it checkedThe fan is not running at the speed the boiler expects, so combustion cannot be verified as safe and the boiler has locked out.Related: fan and flue
- F.35Get it checkedThe air intake or flue is blocked. Something is physically obstructing the path combustion products take out of the building.Related: fan and flue
- F.77Get it checkedThe condensate pump or the flue non-return flap has not responded. The boiler cannot clear its condensate or confirm the flue, so it has shut down.Related: fan and flue
- S.32Get it checkedA protective wait, not a lockout: the fan speed has strayed outside tolerance and the boiler is holding off before it tries to start again. If the wait keeps recurring, F.32 is next.Related: fan and flue
- F.33Stop — call an engineerAn air pressure switch fault: the boiler cannot prove the fan is moving air through the flue. Vaillant check the flue route first, so treat it as a flue fault until proved otherwise.Related: fan and flue
- F.347Get it checkedA 2022 ecoTEC plus has detected condensate inside its combustion chamber and stopped the burner. Vaillant's cause is a blocked condensate discharge — the siphon or the pipe — backing up into the boiler.Related: fan and flue
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Common questions about Vaillant F.334
The boiler is brand new and shows F.334. Is it faulty?
No. It has not been fully commissioned. The flue configuration is a step in the installer's set-up, and the boiler will not run until it is done. Ring the installer; it is their step to finish.
Can I set it myself from the manual?
The setting is in the installer level behind an access code, and Vaillant's operating instructions do not include it. Entering a flue configuration the boiler is not approved for is what F.336 exists to catch, so this is not one to guess at.
It appeared after a repair. Why?
A new board or display arrives without the configuration. Whoever fitted it should have completed the flue and gas settings before leaving, and should come back to do so.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.