Viessmann F5 fault codewhat it means and how to fix it
On the first-generation Vitodens 100 WB1A only, F5 is a faulty air pressure switch. No later Viessmann control unit has an F5 row; if yours shows one, check the type plate.
Take care
Get a Gas Safe registered engineer to look at your Viessmann F5
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 F5 means
F5 belongs to one control unit. The Vitodens 100 and 100-W type WB1A (2004 to 2008) has an air pressure switch that proves the fan is moving air before gas is released, and its service instructions give "F5 — Burner blocked — Air pressure switch faulty — Check the air pressure switch and the interconnecting cable." The burner is blocked, not locked out, so it will try again once the switch proves; a switch that makes intermittently produces an F5 that comes and goes with the fan speed. Every later Vitodens dropped the air pressure switch in favour of reading the fan's own speed signal, and with it the code: the WB1B, WB1C, B1HA/B1KA and B1HC/B1KC tables, the 111-W, the 050-W BPJD and the Vitotronic 200-W tables have no F5 row at all. Their fan faults are F9, FA and FC.
This page previously said F5 was not in the B1HA table and repeated a third-party reading of it as a gas pressure switch; the first part was right and the second is not supported by any Viessmann document we read, so it has gone. If your boiler shows F5 and its type plate does not say WB1A, the display is not showing what you think it is, or the boiler is not a Vitodens — take a photograph of the display and the plate for the contractor. On a WB1A, the householder has nothing to do beyond noting the code; the switch and its cable are inside the casing. On the 2020-on numeric-table units the nearest equivalents are F.349 and F.682/F.683 (air mass flow rate not detected correctly, air mass flow rate sensor faulty).
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Air pressure switch interconnecting cable | Viessmann name the cable alongside the switch. A poor plug connection makes the switch look faulty from the control unit's side. | Common |
| Air pressure switch failed or stuck | A diaphragm switch that has hardened or a tube that has come off it. Replaced with the cable checked at the same time. | Common |
| Fan not producing enough air pressure | A tired fan or a partly blocked flue can leave the switch unmade. On a WB1A this can show as F5 rather than as a fan code. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£60–£140
Time on site
1 hour
A switch and cable within a visit. Parts for a WB1A are increasingly obtained rather than stocked, which adds time rather than cost.
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.
Viessmann Vitodens fan
£150–£300Authorised Viessmann parts distributor
Price checked 28 September 2026
Viessmann air pressure switch (Vitodens 100 WB1A)
£30–£70Authorised Viessmann parts distributor
Price checked 28 September 2026
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Related Viessmann fault codes
- F9Get it checkedFan speed too low during burner start. The control unit will not release gas until the fan proves it is turning fast enough, so the burner has locked out. Engineer job.Related: fan and flue
- F.71Get it checkedFan fault on a 2020-on Vitodens: F.71 and F.371 speed too low, F.72 fan still turning, F.457 sluggish or blocked, F.349 air flow not detected. Burner locked out; an engineer checks the fan unit.Related: fan and flue
- F.540Get it checkedCondensate has backed up into the heat cell of a 2020-on Vitodens; F.176 is the same finding on the 100-W and 111-W. Burner locked out. The condensate trap and drain need clearing by an engineer.Related: fan and flue
- b0Get it checkedFlue gas temperature sensor fault — b0 short circuit, b8 lead break. The burner is blocked until the sensor reads plausibly, because it is part of the flue overheat protection. Engineer job.Related: fan and flue
- FAGet it checkedThe fan did not come to rest when the control unit expected it to — "fan idle state not reached". A lockout on the fan or its control, for an engineer.Related: fan and flue
- FCGet it checkedOn a 100-W, 111-W or 050-W the electrical fan control in the control unit is faulty. On a Vitotronic 200-W the same letters mean a gas train or blocked flue fault. Engineer either way.Related: fan and flue
Own a Viessmann 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.