Vaillant’s own manual describes the F27 fault code as a safety switch-off caused by flame simulation, meaning the electronics have picked up what looks like a flame, or a flame-like electrical signal, at a moment when the gas valve should be closed and no flame should exist.
This points to moisture affecting the electronics, a defective flame monitor, or a gas solenoid valve that’s leaking through slightly rather than sealing fully. This fault is most frequently seen on ecoTEC Pro and Plus models built between 2005 and 2012, though it can appear on newer units too.
A handful of basic checks are safe to try yourself, but the underlying fault always needs a Gas Safe registered engineer.
What Does The Vaillant Boiler F27 Fault Code Mean?
The Vaillant boiler F27 fault code means the printed circuit board is detecting an ionisation current, the electrical signal that normally confirms a flame is present, at a time when there should be no flame at all, typically before the gas valve has even opened. This is sometimes called a false flame or parasitic ionisation fault, and it’s essentially the opposite problem to a boiler that can’t detect a real flame.
Because the control board believes a flame already exists when it shouldn’t, it refuses to begin the normal ignition sequence, since starting the process under these conditions could mean gas is escaping and igniting somewhere it isn’t meant to.
This makes the F27 error quite different from the F28 fault code, which relates to a genuine failure to detect any flame at all during ignition, and from the F29 error code, which covers a flame that was successfully established and then lost partway through a cycle.
What Causes The F27 Fault Code On Your Vaillant Boiler?
The causes of the F27 error code on Vaillant boilers comes down to something creating a false or residual flame signal, whether that’s moisture, a contaminated sensing electrode, a gas valve that isn’t sealing properly, or a genuine PCB fault. Here’s a closer look at each cause.
Moisture On The Flame Sensing Electrode Or Wiring
Moisture on the flame sensing electrode or its wiring is, according to multiple heating engineers, the single most frequent trigger for F27. Even a small amount of condensation, a nearby leak, or general damp reaching the ionisation probe or its connecting cable can create a tiny stray electrical current that mimics the signal a genuine flame would produce.
The control board can’t distinguish between this parasitic current and a real flame, so it assumes a flame is present and refuses to proceed with ignition.
How To Fix
- Press and hold the reset button, usually marked with a flame symbol, for around three seconds to rule out a one-off glitch such as brief condensation.
- If the fault clears and doesn’t return, it’s likely resolved itself as the moisture dried out naturally.
- If F27 returns, check the area beneath and around your boiler for damp patches, drip marks, or corrosion staining on nearby pipework, which can point to a leak contributing to the moisture.
- Do not keep resetting the boiler repeatedly, since this won’t dry out a genuine moisture problem and may mask a leak that needs fixing.
- Book a Gas Safe registered engineer, who will test the ionisation current at the PCB with the gas valve closed. A reading above roughly 0.3 microamps with no flame present confirms a false signal, and they’ll trace and dry the source before checking cable insulation resistance, replacing any wiring that reads below 1 megaohm.
Faulty Or Contaminated Flame Sensing Electrode
A faulty or contaminated flame sensing electrode can produce the same false signal as moisture, even in a perfectly dry boiler. Over years of use, carbon deposits, oxidation, or general wear can affect the probe’s surface in a way that generates a stray current, or the electrode’s ceramic insulator can crack, allowing current to leak where it shouldn’t. Vaillant’s own manual specifically lists a defective flame monitor as a named cause of this fault.
How To Fix
- A Gas Safe engineer will inspect the electrode for carbon build-up, corrosion, or cracks in its ceramic housing.
- They’ll disconnect the electrode and test whether the PCB still reports a flame signal in its absence, which confirms whether the fault lies with the electrode itself or further inside the board.
- If the electrode is contaminated, they’ll clean it thoroughly and retest.
- If it’s cracked or worn beyond cleaning, they’ll replace it with a genuine manufacturer part.
- They’ll run the boiler through several ignition cycles afterwards to confirm no false flame signal is present before start-up.
If your boiler needs this kind of ignition component testing and replacement more than once every year or two, it’s worth thinking about how those repeat visits add up in cost over time, particularly on an older system.
Clever Energy Boilers offers free online boiler quotes, so it’s a simple way to compare the ongoing cost of repairs against a fresh, more dependable installation.
Gas Valve Leaking Through Or Closing Too Slowly
A gas valve that’s leaking through or closing too slowly can allow a tiny amount of gas to reach the burner when it shouldn’t, occasionally causing a small, unintended flicker of flame or combustion residue that the sensor picks up as a genuine flame signal. This is one of the more serious causes of the F27 error, since it relates directly to the valve’s ability to seal gas flow completely when required.
How To Fix
- This fault must only be diagnosed and repaired by a Gas Safe registered engineer, since it involves internal gas safety components.
- The engineer will test the gas valve’s sealing performance, checking for any gas passing through when the valve should be fully closed.
- They’ll measure how quickly the valve closes during a normal shutdown sequence to rule out a slow-closing mechanism.
- If the valve is confirmed to be leaking through or closing too slowly, they’ll replace it entirely, since this component isn’t typically repaired individually.
- They’ll carry out a full gas tightness test and several ignition cycles afterwards to confirm the fault has been fully resolved.
Damaged Or Damp Ignition Wiring Or Connectors
Damaged or damp wiring and connectors relating to the ignition and flame sensing circuit can create exactly the same false flame symptom as a faulty electrode, since a compromised cable can pick up or generate stray electrical signals just as easily as the sensor itself. This is particularly common in boilers where condensate has previously leaked near the wiring loom, gradually causing corrosion at the connector pins over time.
How To Fix
- A Gas Safe engineer will visually inspect the wiring harness and connectors running from the flame sensing electrode back to the PCB.
- They’ll check for any signs of moisture damage, corrosion, or physical damage to the cable insulation.
- They’ll test the wiring’s insulation resistance, replacing any section that reads below the expected safe threshold.
- If a source of moisture, such as a nearby condensate leak, is found to be contributing to the damage, they’ll trace and repair that at the same time.
- They’ll retest the boiler through multiple ignition cycles to confirm the fault has cleared consistently.
Faulty PCB Flame Amplifier
A faulty PCB flame amplifier is a less common but still recognised cause of F27, and it’s usually confirmed only once the electrode, wiring, and gas valve have all tested correctly. The flame amplifier is the specific part of the circuit board responsible for interpreting the ionisation signal from the sensor, and if this internal component has failed, it can report a flame signal that simply doesn’t exist anywhere in the physical system.
How To Fix
- The engineer will disconnect the flame sensing electrode entirely and check whether the PCB still reports a flame signal despite nothing being physically connected.
- If the board continues to show a flame signal with the electrode disconnected, this confirms an internal fault with the PCB’s amplifier circuit rather than any external component.
- They’ll replace the PCB, since this specific amplifier fault isn’t something that can be repaired in isolation.
- They’ll reconfigure or recalibrate the system as needed following the replacement.
- A full commissioning check will follow to confirm the false flame signal no longer appears during start-up.
False Flame Faults Keep Locking Out Your Boiler? Get A Free Quote From Clever Energy Boilers
An F27 fault that keeps returning, particularly one traced back to a leaking gas valve or a failing PCB, often points to a boiler where core safety components are reaching the end of their reliable working life. This is especially true on ecoTEC models from the 2005 to 2012 era, which are now well past a decade old and increasingly likely to need one ignition related repair after another as parts age together.
Given that boilers typically last between 10 and 15 years, a system from this period showing recurring flame simulation faults is often better replaced than repeatedly repaired, since a PCB replacement alone can be a substantial cost on a boiler already nearing retirement.
At Clever Energy Boilers, we install dependable, efficient boilers for homeowners and landlords across the North West, Yorkshire, and the Midlands, helping people move away from unreliable ignition and flame detection faults for good.
Request your free, no-obligation boiler quote today.
A new boiler installation means a fresh flame sensing system, gas valve, and control board working together correctly from day one, backed by a full manufacturer warranty, along with considerably better efficiency than an older unit approaching the end of its lifespan. Our engineers will always give you a clear, honest comparison between repairing your current boiler and investing in a new, reliable replacement.
Frequently Asked Questions
Can F27 Clear Itself Without Any Repair?
F27 can occasionally clear itself after a single reset if it was caused by something transient, such as brief condensation on the electrode that dries out naturally. However, if the fault returns after that one reset, it indicates a genuine underlying issue that will need a Gas Safe engineer to properly diagnose and resolve.
Is F27 More Serious Than A Standard Ignition Fault Like F28?
F27 is generally considered at least as serious as, and in some respects more concerning than, a standard ignition failure like F28, since it can point towards a gas valve that isn’t sealing correctly, which has direct gas safety implications. It should always be treated as a fault requiring prompt professional attention rather than something to keep resetting.
Why Is F27 More Common On Older Vaillant Boilers?
F27 is more commonly reported on ecoTEC models built between 2005 and 2012 largely because the ignition electronics, wiring, and gas valves on boilers of that age have simply had more years to develop wear, corrosion, or moisture related faults compared with newer units built with updated components.
Can A Recent Power Cut Cause F27 To Appear?
Yes, a recent power cut or an unexpected boiler reset can occasionally contribute to F27 appearing, since a sudden loss and return of power can sometimes cause the electronics to briefly misread a residual signal as a flame. If the fault clears after one reset following a power interruption, it’s likely related to this rather than a genuine component fault.
Does F27 Always Mean The PCB Needs Replacing?
No, F27 doesn’t always mean the PCB needs replacing. In many cases, the fault is caused by something simpler, such as moisture on the electrode or damaged wiring, both of which can be resolved without any electronic component being replaced at all. A PCB replacement is usually only confirmed as necessary once every other possible cause has been tested and ruled out.
