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.27 fault codewhat it means and how to fix it
The boiler is detecting a flame when the burner should be off. Switch it off and get a Gas Safe engineer out rather than resetting it.
Stop — this needs an engineer
Get a Gas Safe registered engineer to look at your Vaillant F.27
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.27 means
Vaillant describe F.27 as a flame signal detected while the burner is off, and label it a safety switch-off for flame simulation. There are two ways that can happen and they are not equally serious. The likely one is that the flame detection circuit is producing a false reading — the ecoTEC tables list "moisture on the electronics" and "electronics (flame monitor) defective" — where contamination on the electrode, moisture across a connector, or a degraded amplifier on the board creates a signal that mimics a flame.
The serious one is that the reading is correct and gas is genuinely still burning after the valve was commanded shut, which means the gas valve is passing; Vaillant do not leave that to inference, because "gas solenoid valve leaking" is the third cause in every ecoTEC table since 2005 and the first one on their UK fault-code page. Neither you nor the appliance can distinguish between them without test equipment, so the only sensible course is to treat it as the serious possibility until an engineer has proved otherwise. This is one of a small number of boiler codes where the right first move is to isolate the appliance rather than reset it and see what happens.
One older range means something else by the same number. On the ecoMAX 600 and 800 series and the turboMAX plus and thermoCOMPACT, F.27 is "no demand to gas valve" — the board is not driving the valve at all, with a defective electronic board, a defective gas valve, or no or insufficient gas as the listed causes. That is a boiler that will not light rather than one that may be burning when it should not, but it is the same component and the same engineer.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Flame monitor electronics defective | Named directly in the Vaillant tables as "electronics (flame monitor) defective". Contamination or a wiring breakdown on the ionisation electrode produces a signal that looks like a flame. The most common cause and the benign one. | Common |
| Gas solenoid valve leaking | Vaillant's own words, in the ecoTEC tables and on their fault-code page. The valve is not sealing fully when closed, so gas continues to reach the burner. Less common, and the entire reason this code is handled as urgent. | Occasional |
| Moisture tracking across the electrode lead | Water from a leak inside the case, or condensation in an unheated garage or loft installation, providing a conductive path. | Occasional |
| Control board flame amplifier fault | The board misreads its own input. Diagnosed once the gas side has been proved sound. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£130–£420
An electrode or flame detector is £130 to £220 fitted. A gas valve is £280 to £420. Expect a diagnostic charge on top, because separating the two needs a flue gas analyser.
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 ignition and sensing electrode
0020039221£19–£46Authorised Vaillant parts distributor
Price checked 28 September 2026
Vaillant ecoTEC gas valve
0020053968£110–£210Authorised 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.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: ignition and flame
- F.29Stop — call an engineerThe boiler lit correctly and then lost the flame while it was running. It has shut the gas off and locked out, and it needs an engineer.Related: ignition and flame
- F.68Get it checkedThe flame is lighting but its signal is unstable. Vaillant's causes run through the gas supply, the air ratio, the condensate route and the flue, ending with the electrode and the board.Related: ignition and flame
- F.87Get it checkedThe ignition transformer, or on the ecoFIT pure the electrodes, are not connected as the board expects. A wiring fault on the ignition side; the boiler will not attempt to light.Related: ignition and flame
- 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
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Common questions about Vaillant F.27
What should I actually do right now?
Switch the appliance off at its fused spur rather than holding the reset button and carrying on. If you can reach the gas isolation valve serving the boiler without removing any panels, close that too, and open a window in the room. Then book a Gas Safe registered engineer and tell them the appliance is reporting a flame signal with the burner off — those words get it prioritised properly rather than queued as a breakdown.
Should I be worried about carbon monoxide?
It is worth checking that any alarm in the property is working and in date, which you should be doing anyway. But treat the alarm as a backstop, not a reason to keep using the appliance. If anyone feels sick, dizzy or drowsy, or you can smell gas at any point, leave without operating any electrical switch and call 0800 111 999 from outside.
It cleared after a reset and seems fine. Can I leave it?
No. A gas valve that passes intermittently is more dangerous than one that has failed outright, precisely because it gives you this reassurance. The display clearing does not mean the fault has gone.
Realistically it is probably just the electrode, though?
Statistically that is the more likely of the two, yes. But the cost of being wrong is a gas escape and the cost of being cautious is one cold night, and that asymmetry is why the advice is to isolate rather than to back the odds.
Is this a carbon monoxide risk?
The immediate concern is unburnt gas rather than combustion products. But any fault where a gas valve may not be closing properly is a good reason to make sure there is a working CO alarm in the property, and if there is not one, fit one whatever the outcome here.
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 support portal
- Manufacturer technical and service documentation
- Trade body guidance
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.