Vaillant F.55 fault codewhat it means and how to fix it
The CO sensor — Vaillant's "all-gas sensor" that lets the ecoTEC exclusive regulate its own combustion — is defective or short-circuited. The boiler will not run without it.
Take care
Get a Gas Safe registered engineer to look at your Vaillant F.55
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.55 means
The ecoTEC exclusive, and the ..6/5-5 plus table that shares its rows, carry a combustion-regulating electronics package that no earlier Vaillant had. Part of it is a sensor Vaillant call the all-gas sensor, which reads the combustion products so the board can trim the gas valve for the fuel actually arriving.
F.55 is that sensor failing. The plus table prints "fault: CO sensor — all-gas sensor defective or has short-circuited; check plug, cable harness, all-gas sensor, PCB", and the exclusive table reads as a repair sequence: check the cable harness; all-gas sensor defective, replace the all-gas sensor; electronics defective, replace the PCB. The row is on the exclusive and the ..6/5-5 plus tables only — not the pro, the ecoFIT, the ecoTEC sustain or the 2022 CS/1-5, whose combustion control works from the ionisation electrode instead.
Despite the name, F.55 is not a warning that carbon monoxide is in the room. It is the boiler reporting that the instrument it uses to keep its own combustion clean has stopped answering, and refusing to run on an unmeasured mixture. That is the protection working as designed, and it is entirely an engineer's repair.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| All-gas sensor failed or shorted | Vaillant's named cause. The sensor element has broken down, and the board reads a short or nothing at all. | Common |
| Plug or cable harness fault | The first check on both tables. A loose or damaged connection between the sensor and the board reads the same as a failed sensor. | Common |
| Electronics defective | The last item on the exclusive list. The board's sensor input has failed and a good sensor on a sound harness still reads F.55. | Rare |
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 harness fault is a call-out of £90 to £150. The sensor and its fitting are typically £180 to £300; a board £250 to £400.
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.57Get it checkedThe combustion regulation's measuring programme has found a fault — on the exclusive, a badly corroded ignition electrode; on the 2022 CS/1-5, F.057 means the regulation and its fallback mode have both failed.Related: combustion control
- F.56Stop — call an engineerA safety shutdown because the boiler's combustion regulation measured carbon monoxide in the flue gas above its limit. Vaillant say that if it comes back after a reset, the gas valve is defective.Related: combustion control
- F.93Get it checkedVaillant print F.93 as "check gas route": combustion quality is outside the permitted range. The causes are the gas injector, the gas group, flue gas recirculation and a blocked pressure point in the venturi.Related: combustion control
- F.22Get it checkedThe boiler has shut down because there is not enough water in the system. Pressure has dropped below roughly 0.5 bar and it will not fire until you top it up.Related: same brand
- F.75Get it checkedThe boiler cannot detect a rise in pressure when the pump starts, so it has locked out. Usually a failed pressure sensor or a pump that is not moving water.Related: same brand
- F.28Stop — call an engineerThe boiler tried to light and failed every attempt it is allowed — three on an ecoTEC, five on the 2022 CS/1-5 — so it has shut the gas off and locked out.Related: same brand
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Common questions about Vaillant F.55
Is there carbon monoxide in my house?
F.55 does not say that. It says the boiler's own combustion sensor has failed, and the boiler has stopped rather than run unmeasured. If your CO alarm is sounding, that is a different matter: leave, ventilate and call 0800 111 999. If it is silent, F.55 is a repair, not an emergency.
Can the boiler run without the sensor?
No. On these models the sensor is part of how the burner is regulated, and Vaillant's table has it replaced rather than bypassed.
Why is F.55 not in my ecoTEC pro or ecoFIT manual?
Because those boilers have no all-gas sensor. The row belongs to the exclusive's combustion-regulating electronics and to the ..6/5-5 plus table that prints the same rows.
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.