Vaillant F.343 fault codewhat it means and how to fix it
The air-mass flow sensor on a 2022 ecoTEC plus is giving a signal the board does not believe. Vaillant's causes are its plugs and harness, a dirty or failed sensor, or the PCB.
Take care
Get a Gas Safe registered engineer to look at your Vaillant F.343
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.343 means
F.343 is the companion to F.342. Where F.342 is an air-mass reading that is real but outside the limits, F.343 is "the signal for the air-mass flow sensor is not plausible" — a reading the board rejects as impossible.
Vaillant's list is the sensor's own circuit and condition: the air-mass flow sensor plug loose or not plugged in; the plug on the PCB; an interruption in the wiring harness; "measurement discrepancy too high — air-mass flow sensor dirty: check the air-mass flow sensor for dirt"; "you exit the measuring range permanently — air-mass flow sensor defective: replace the entire air intake pipe"; and finally the PCB. The dirty-sensor line is the one with a story behind it. The sensor sits in the air intake, and everything the fan draws in passes it: dust from a workshop or a utility room, fluff from a tumble dryer sharing the space, cobwebs and insects from an outside terminal. A film on the sensing element skews the signal until the board stops believing it.
The sensor is part of the multiple-flue conversion set — the air intake pipe with a built-in non-return valve — so it is fitted where the boiler shares a flue, and it is replaced with the pipe rather than on its own. Unlike F.342, Vaillant's list for F.343 does not include the flue itself, so the page does not carry the CO flag; the boiler has simply lost a measurement it needs and stopped.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Sensor plug or PCB plug loose | Vaillant's first two items. Connectors on the air intake disturbed during a service or a filter clean. | Common |
| Air-mass sensor dirty | Vaillant's "measurement discrepancy too high". Dust or fluff drawn in with the combustion air coats the element; the check is to inspect it for dirt. | Common |
| Wiring harness interrupted | A break in the lead between the sensor and the board. | Occasional |
| Sensor out of range permanently, or PCB defective | Vaillant's last two lines: replace the entire air intake pipe, or replace the PCB. Reached when the plugs, harness and cleaning have not restored a plausible signal. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£90–£400
Time on site
1 hour
A plug reseat or a sensor clean is a call-out of £90 to £150. The air intake pipe with its sensor is priced on the part; a PCB is £250 to £400 fitted.
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
Vaillant wiring harness (plug-and-cable set to a sensor or actuator)
£20–£65Authorised Vaillant parts distributor
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
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.343
Can I clean the sensor?
It is inside the boiler's air intake, which Vaillant restrict to a competent person, and the element is delicate. An engineer inspects it as part of the F.343 check.
How is F.343 different from F.342?
F.342 is a real air-flow measurement outside the limits — the fan or the flue. F.343 is a signal the board does not believe at all — the sensor or its wiring. The engineer's starting point is different for each.
Why do I keep getting it in a dusty room?
Because the boiler breathes that room's air unless its intake comes from outside through the flue. Keeping the space clean, or having the installer check the air supply arrangement, addresses the cause rather than the symptom.
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.