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.56 fault codewhat it means and how to fix it
A 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.
Stop — this needs an engineer
Get a Gas Safe registered engineer to look at your Vaillant F.56
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.56 means
F.56 is the one Vaillant code that names carbon monoxide as the thing measured. The ecoTEC exclusive and the ..6/5-5 plus table print it as "safety switch-off: CO limit exceeded — safety shutdown after the CO limit value is exceeded", and both give the same two-part reading.
First, "a component in the combustion regulation is defective", and the specific suspect is a contact fault at the gas valve assembly: the plug not seated or not connected, the plug defective, or the slot on the valve loose. Second, and in Vaillant's own words, "if the fault occurs again after being reset: the gas valve assembly is defective". The boiler measures the CO in its own flue gas with the all-gas sensor, and a reading above the limit means the burner is producing more CO than a correctly regulated flame should — the mixture has drifted rich because the valve is not doing what the board asks.
The shutdown is the protection working: the products are in the flue, not the room, and the boiler has stopped making more. But it is a combustion fault by definition, it earns the carbon monoxide flag on this site, and Vaillant give no householder action beyond the single reset their table refers to. It is not a row on the pro, ecoFIT, ecoTEC sustain, CS/1-5 or older tables, none of which measure CO.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Gas valve not regulating — valve defective | Vaillant's conclusion if the code returns after one reset. The valve is passing more gas than the board is calling for and the mixture runs rich. | Common |
| Contact fault at the gas valve plug | The exclusive table's first suspect: the valve plug not pushed home, a defective plug, or a loose slot on the valve. The board loses fine control of the valve without losing it entirely. | Common |
| Cable harness fault | On the plus table's check list with the valve and the plug. A damaged harness between board and valve produces the same loss of regulation. | Occasional |
| Combustion regulation component failed | Vaillant's general heading. A fault elsewhere in the sensing and regulating chain that lets the mixture drift before the limit catches it. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£120–£400
Time on site
1h 30m
A plug or harness repair with a combustion check is £120 to £180. A gas valve assembly is £280 to £400 fitted.
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 gas valve
0020053968£110–£210Authorised 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.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.55Get it checkedThe 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.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.56
Is my family in danger?
The boiler measured the CO in its flue, where it belongs, and shut down. Nothing in F.56 says CO has entered the room. Treat it seriously all the same: reset once at most, do not run the boiler if the code returns, check your CO alarm is working and book a Gas Safe engineer. If an alarm sounds or anyone has headaches or nausea, leave the house and call 0800 111 999.
Vaillant say reset it once. Then what?
Their table is explicit: if F.56 occurs again after being reset, the gas valve assembly is defective. A second appearance is the diagnosis, not a reason for a third attempt.
Will the engineer just replace the valve?
They will check the valve's plug and harness first, because a loose connection produces the same code, and then replace the valve if the code persists. Expect a combustion analysis afterwards to confirm the boiler is regulating again.
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.