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.33 fault codewhat it means and how to fix it
An 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.
Stop — this needs an engineer
Get a Gas Safe registered engineer to look at your Vaillant F.33
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.33 means
F.33 belongs to the Vaillant boilers that carry an air pressure switch or sensor to prove air flow, which is not all of them. The ecoTEC exclusive prints it as "pressure switch" or, in the later edition, "air/flue gas pressure implausible", with a check list that runs through the cable harness, the vacuum hose for blockage, the supply air and flue gas route for blockage, the panel being the correct type, the flue pipe length, the air pressure sensor, the D.132 multiple-flue setting, the pressure switch and the fan. The 48–64 kW ecoTEC plus puts it in two lines: "air/flue pipe blocked — check the entire air/flue pipe; defective air pressure switch — replace".
On the turboMAX plus 837 and 637 the same number is the older switch: "no switching signal from the air pressure switch — check air pressure switch, fan, fan supply or flue", with the note that a replacement sensor must have a switching point of 105/90 Pa. The ecoTEC plus ..6/5-5, pro, ecoFIT and ecoTEC sustain tables have no F.33 row, because those boilers prove the fan by its speed signal (F.32) rather than a pressure switch.
What matters to a householder is the order of Vaillant's list: the flue comes before the switch. A blocked flue or air intake is one of the few boiler faults that can put combustion products where they should not be, and Vaillant's consumer page adds "blockage in the flue or condensate discharge" among its causes. This page carries no steps for that reason.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Air or flue route blocked or restricted | Vaillant's "supply air/flue gas route (blockage)" and the 48–64 kW table's first cause. Debris or a nest at the terminal, a crushed or displaced flue section, or ice at the terminal in winter. | Common |
| Vacuum hose blocked or detached | The small tube that carries pressure to the switch or sensor. Blocked with condensate or debris it reports no flow with the fan running normally. | Common |
| Air pressure switch or sensor failed | The switch itself. Vaillant list it after the flue checks, and the turboMAX plus specifies the 105/90 Pa switching point for its replacement. | Occasional |
| Fan, wrong panel or flue length outside the setting | The rest of the exclusive list: a fan not reaching speed, a front panel of the wrong type disturbing the air path, a flue longer than the D.132 configuration allows. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£90–£320
Time on site
1 hour
Clearing a flue terminal or replacing a hose is £90 to £160. An air pressure switch is £120 to £200 fitted; a fan £230 to £320. Flue rebuilding is priced on inspection.
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.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
- F.334Get it checkedA 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.Related: fan and flue
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Common questions about Vaillant F.33
Can I just reset it and see?
One reset does no harm, but if F.33 returns stop there. The boiler is refusing to fire because it cannot prove its flue is clear, and forcing repeated attempts is the wrong response to that. Book a Gas Safe engineer and make sure your CO alarm is working.
Why does my ecoTEC plus manual not list F.33?
The ..6/5-5 plus, the pro, the ecoFIT and the ecoTEC sustain prove air flow from the fan's speed signal and report a problem as F.32. F.33 appears on the exclusive, the 48–64 kW plus and the turboMAX plus 837 and 637, which have a pressure switch or sensor.
Is it a fan fault or a flue fault?
Either, and Vaillant's list has the engineer look at the flue first. Only when the air route, the hose and the flue length check out does the switch or the fan get replaced.
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.