The F27 fault code means the Vaillant boiler has detected a flame signal while the burner is switched off. In simple terms, the control board thinks a flame is present when it should not be.
This is a safety lockout designed to stop any risk of unburnt gas or unsafe operation. The code appears on many ecoTEC models used in UK homes and by landlords. Heating and hot water stop until a Gas Safe engineer diagnoses and clears the fault.
What Does The Vaillant Boiler F27 Fault Code Mean?
The Vaillant boiler F27 fault code means a flame signal has been detected while the burner is off. Your boiler constantly checks for the presence of a flame using a sensing electrode. When that electrode reports a flame at a time when the gas valve should be fully closed, the control board shuts everything down.
Official Vaillant guidance lists the main causes as a gas solenoid valve that is not leaktight or a defective printed circuit board. This is not a fault you can safely ignore or repeatedly reset. Understanding the meaning helps you know why the appliance is protecting itself and why professional help is essential.
What Causes The F27 Error Code On Your Vaillant Boiler?
The F27 error code on your Vaillant boiler is caused by the control board receiving a flame signal when the burner is not supposed to be alight. The most common reasons are a gas valve that is not sealing properly, a dirty or faulty flame sensing electrode, moisture affecting electrical parts, or a defective printed circuit board.
These issues are more common on older ecoTEC models and boilers that have not been serviced regularly. Below we explain each main cause in everyday language and show the correct steps to resolve them. Because this fault involves gas safety, all work beyond a single reset must be carried out by a Gas Safe registered engineer.
Gas Valve Not Sealing Properly
A gas valve that is not sealing properly is one of the most serious causes of the F27 fault code. The gas solenoid valve is designed to open only when the boiler needs to fire and to close tightly the rest of the time. If the valve fails to close fully, a tiny amount of gas can pass through. The flame sensing electrode then detects a signal even though the burner should be off, and the boiler locks out for safety.
This can happen through natural wear, dirt inside the valve, or age. You may notice the fault appears soon after the heating or hot water demand ends.
How To Fix
- Do not attempt to open the boiler or touch the gas valve yourself. This is gas work and must be left to a qualified person.
- Switch the boiler off at the isolation switch if you can do so safely.
- Note any smell of gas. If you smell gas, leave the property immediately and call the National Gas Emergency number 0800 111 999.
- Contact a Gas Safe registered engineer who works with Vaillant boilers.
- The engineer will test the gas valve for leakage, check the wiring to the valve and replace the valve if it is not sealing correctly. After replacement they will carry out a full safety and combustion check.
Faulty Or Dirty Flame Sensing Electrode
A faulty or dirty flame sensing electrode frequently triggers the F27 error. The electrode sits in the combustion chamber and reports whether a flame is present. Over time it can become coated with soot or carbon, the ceramic insulator can crack, or the electrode tip can wear.
Any of these problems can send a false “flame present” signal to the control board even when the burner is off. The result is an immediate lockout. This is a common finding on boilers that have not been serviced for several years.
How To Fix
- Do not open the boiler casing. The electrode sits inside the sealed combustion chamber.
- Make a note of when the F27 appears (for example, after the heating has just switched off).
- Call a Gas Safe registered engineer.
- The engineer will remove, inspect, clean or replace the flame sensing electrode and check the associated wiring.
- After the repair they will test the flame signal under normal operating conditions and confirm the boiler runs safely.
Moisture On Electrical Components Or The PCB
Moisture on electrical components or the printed circuit board is a frequent cause of the F27 fault. Condensation can form inside the boiler, especially if the condensate trap is not draining properly or if there has been a small water leak. Water or dampness on the PCB or the flame sensing wiring can create false signals that the control board interprets as a flame. This problem is more common on boilers fitted in colder or poorly ventilated cupboards and on older models where seals have aged.
How To Fix
- Look for any obvious signs of water around the base of the boiler or on the floor, but do not touch internal parts.
- Check that the condensate pipe is clear and not frozen (especially in cold weather).
- Reset the boiler once only, following the instructions in your handbook, to see if the fault clears temporarily.
- Arrange a visit from a Gas Safe engineer.
- The engineer will dry the affected components, check for the source of moisture, clean or replace damaged wiring or the PCB if necessary, and re-test the flame detection circuit.
Defective Printed Circuit Board
A defective printed circuit board can produce the F27 code even when the gas valve and electrode are working correctly. The PCB is the electronic brain of the boiler. If it develops a fault it can misread the signal coming from the flame sensing electrode and decide that a flame is present when it is not.
PCB problems sometimes appear after a power surge, moisture damage or simply with age. Replacing the board is a more expensive repair and is usually only done once the engineer has ruled out the gas valve and electrode.
How To Fix
- Switch the boiler off at the isolation switch.
- Do not attempt any further resets once the code returns.
- Contact a Gas Safe registered engineer experienced with Vaillant boilers.
- The engineer will carry out diagnostic checks on the flame sensing circuit, gas valve and PCB.
- If the PCB is confirmed faulty they will replace it with the correct Vaillant part, re-programme any settings if required, and carry out a full commissioning check.
If the Vaillant boiler F27 fault code keeps returning after these repairs, the underlying system may simply be reaching the end of its reliable life. Continuing to repair an ageing boiler can become more expensive than upgrading. A modern high-efficiency boiler often cuts energy use and comes with a long manufacturer warranty, giving both homeowners and landlords peace of mind.
How Can You Prevent The Vaillant Boiler F27 Fault Code In Future?
Preventing the F27 fault code starts with regular professional care. Book an annual Gas Safe service so an engineer can inspect the gas valve, clean the flame sensing electrode and check for any signs of moisture. Keep the area around the boiler dry and well ventilated. Ensure the condensate pipe is clear and insulated against freezing. Avoid repeated power cycling of the boiler when a fault appears. These simple habits reduce the chance of false flame signals and keep the safety systems working as designed.
Replace Recurring Vaillant F27 Faults with a Reliable New Boiler
If your Vaillant boiler F27 fault code keeps returning, it could be a sign that key safety components such as the gas valve or control board are reaching the end of their service life. While repairs may resolve the issue temporarily, repeated breakdowns often become more expensive than replacing an ageing boiler.
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FAQs
Is the Vaillant boiler F27 fault code dangerous?
Yes, it is a safety-related lockout. It means the boiler has detected a flame signal when the burner should be off, which can point to a gas valve that is not sealing. Do not ignore it or keep resetting. Call a Gas Safe engineer promptly and if you smell gas leave the property and dial 0800 111 999.
Can I reset the Vaillant boiler F27 fault code myself?
You can try one reset after checking for any gas smell. If the code returns immediately or reappears the next time the boiler tries to fire, stop resetting and arrange professional help. Repeated resets will not fix the underlying problem and can mask a serious fault.
Does a landlord need to act quickly when a tenant reports the Vaillant boiler F27 fault code?
Yes. Landlords have a legal duty to keep heating and hot water systems safe. An F27 lockout leaves the property without heat, so a Gas Safe engineer should attend as soon as possible. Keep clear records of the call-out and any work carried out.
Is the F27 fault more common on certain Vaillant models?
It appears most often on older ecoTEC Pro and ecoTEC Plus boilers, particularly those manufactured between roughly 2005 and 2012, although it can occur on later models as well. Regular servicing reduces the likelihood on any age of boiler.
Will replacing the flame sensing electrode always clear the F27 code?
Not always. While a dirty or worn electrode is a common cause, the engineer must also check the gas valve and the PCB. Replacing only the electrode when the gas valve is leaking will not solve the problem. A proper diagnosis is essential.