Vaillant F.37 fault codewhat it means and how to fix it
A turboMAX plus 837 or thermoCOMPACT 637 code only: the fan is turning too fast or too slow for the board, and Vaillant blame the air pressure switch, the fan or the electronics.
Take care
Get a Gas Safe registered engineer to look at your Vaillant F.37
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.37 means
F.37 appears in one Vaillant table, the turboMAX plus and thermoCOMPACT ../2 E installation and servicing instructions, and there it is marked "837/637 only": "the rotary speed of the fan is too high or too low — air pressure switch defect; fan defect; electronics defect". The 37 kW models of those two non-condensing ranges were the only ones with a speed-controlled fan and an air pressure switch proving airflow, which is why they alone carry F.32, F.33, F.37 and F.41 and why a smaller 824 or 628 will never show any of them.
The three fan rows divide the problem up: F.32 is the heat exchanger anti-freeze mode entered because the speed deviation is too great, F.33 the same mode entered because the air pressure switch gave no switching signal, and F.37 the plain statement that the measured speed is outside the band the board expects, with the switch, the fan and the board as the three suspects.
The later ecoMAX and every ecoTEC fold all of this into a single F.32 fan fault, so an owner reading about F.37 elsewhere will usually find it described for a different boiler; on a Vaillant it is a turboMAX plus 837 E or a thermoCOMPACT 637 E, sold between 1999 and 2005, and nothing else. The fan sits in the sealed combustion path, so the checks are an engineer's.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Fan motor worn | On a boiler of this age the fan bearings and the speed pick-up are the usual reason the measured speed drifts out of band. Vaillant list the fan second. | Common |
| Air pressure switch faulty or its tubes blocked | Vaillant's first suspect. The switch proves airflow; a split sensing tube, a switch stuck open or closed, or condensation in the tubes confuses the board's picture of what the fan is doing. | Common |
| Control board | The third listed cause. The fan speed control lives on the main PCB, which on an 837/637 is now more than twenty years old. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£320
Time on site
1 hour
A diagnostic visit is £80 to £120. An air pressure switch is £60 to £120 fitted; a fan, where one can still be sourced for a 2001 boiler, £220 to £320.
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 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
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.