This fault can involve incomplete combustion and carbon monoxide.
If anyone in the property feels sick, dizzy or drowsy, or you can smell gas: open the windows, leave the building, and do not switch anything on or off. Then call the National Gas Emergency Service.
0800 111 999Vaillant F.342 fault codewhat it means and how to fix it
The 2022 ecoTEC plus measures the air its fan moves, and F.342 means that air-mass flow is outside the permitted limits. Vaillant's list runs from fan speed and plugs to a blocked flue.
Take care
Get a Gas Safe registered engineer to look at your Vaillant F.342
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.342 means
Two-digit Vaillants prove their fan by its speed. The ecoTEC plus CS/1-5 goes further: on installations with the multiple-flue conversion set — an air intake pipe with a built-in non-return valve and an air-mass flow sensor — it measures the air actually arriving, and F.342 is that measurement falling outside limits. Vaillant's check list works from the fan outward. First, "fan speed does not correspond to the target value — use D.033 and D.034 to check whether the fan speed deviates by more than 20 to 30 rpm".
Then the electrical path: the plug on the PCB, the wiring harness, the plug on the air-mass flow sensor. Then the sensor — "you exit the measuring range permanently; air-mass flow sensor defective: replace the entire air intake pipe", because the sensor is built into the pipe and is not changed separately. And last, "flue gas pipe blocked — check the entire flue gas pipe".
The sister code F.343 is the same sensor giving an implausible signal rather than an out-of-range one. That last cause is why this page carries the carbon monoxide flag: a flue restriction that reduces the air the boiler can draw is exactly what an air-mass measurement is there to catch, and Vaillant put it on their own list. There is nothing here for a householder to do; the D. codes are in the installer level and the sensor is inside the air intake.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Fan speed off target | Vaillant's first check, read against D.033 and D.034. A fan not reaching the speed the board asks for moves less air than the sensor expects. | Common |
| Sensor or PCB plug, or harness | Three of Vaillant's six items are the electrical path between the air-mass sensor and the board. Disturbed connectors after work on the air intake. | Common |
| Flue gas pipe blocked | Vaillant's last cause and the one that matters most. A restricted flue reduces the air flow the boiler can achieve at any fan speed. | Occasional |
| Air-mass flow sensor defective | The sensor has left its measuring range permanently. Replaced with the whole air intake pipe it is built into. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£90–£380
Time on site
1 hour
A plug or harness fault is a call-out of £90 to £150. A fan is £230 to £320 fitted; the air intake pipe with its sensor is priced on the part. Flue work depends on what is found.
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 ecoTEC fan assembly
0020039840£120–£230Authorised 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.342
Why the carbon monoxide flag?
Because a blocked flue is on Vaillant's own cause list for F.342, and a flue problem is the one class of fault that can put combustion products where they should not be. The boiler has stopped; keep it stopped until an engineer has checked the flue, and make sure your CO alarm works.
Does every CS/1-5 have this sensor?
The air-mass flow sensor comes with the multiple-flue conversion set, so it is fitted where the boiler shares a flue system. A boiler on its own flue may never show F.342 or F.343.
Is the sensor a small part?
No — Vaillant replace the entire air intake pipe with the sensor built in. Expect that to cost more than a plug-in sensor would.
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.