Vaillant F.27 Fault Code: Causes, Fixes & Repair Costs
What does the Vaillant F.27 fault code mean?
The F.27 code appears on Vaillant ecoTEC Pro and Plus boilers when the boiler's control system detects what it considers an incorrect flame signal — either a flame is sensed when there should be none, or the flame isn't confirmed when the burner fires. Vaillant refer to this internally as a 'flame simulation' safety shutdown. The result is a full boiler lockout: your heating and hot water stop working until the underlying fault is identified and resolved. The code is most frequently encountered on ecoTEC models built between 2005 and 2012, though it can appear on newer units too.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Moisture on electrical components Common
Water finding its way onto the boiler's electronics is the single most frequent trigger for F.27. This can happen through a slow internal leak — for example from the heat exchanger or a corroded pipe joint — allowing droplets to reach the control board or flame-monitoring circuitry. Even condensation in a poorly ventilated boiler cupboard can be enough to cause intermittent faults.
- Printed circuit board (PCB) failure Common
The PCB is responsible for interpreting the flame sensor signal, and when it begins to fail the readings become unreliable. A telltale sign of PCB trouble on Vaillant ecoTECs is the boiler cycling through different fault codes — F.27, F.61, F.62 — on successive resets rather than consistently showing the same one. Failing transformers on the board can alter supply voltages in ways that mimic a flame-simulation error.
- Faulty or contaminated flame sensor (ionisation electrode) Sometimes
The flame sensor passes a small electrical current through the burner flame to confirm ignition. If the electrode is coated in combustion deposits, has drifted out of position, or has cracked, it can report a false signal to the PCB — either claiming a flame exists when the burner is off, or failing to confirm ignition when gas is burning correctly.
- Faulty or leaking gas valve Sometimes
The gas valve controls how much gas enters the combustion chamber. If it fails to close fully, a small residual flame can persist after shutdown — which the boiler interprets as a flame-simulation fault. Equally, a valve that sticks partially closed can produce an unstable flame that the sensor cannot reliably confirm.
- Worn ignition leads or damaged spark electrode Sometimes
Cracked ignition leads or a worn spark electrode can cause incomplete or inconsistent ignition. The boiler fires, the gas briefly ignites but the flame isn't stable enough for the sensor to confirm it properly, triggering a lockout. Repeated failed ignition attempts can leave unburnt gas in the chamber, which also contributes to the flame-simulation error.
- Interrupted or low gas supply Rare
If gas pressure at the meter is low, or the gas isolation valve has been inadvertently partially closed, the boiler may fail to sustain a proper flame. Severe weather can occasionally cause pressure fluctuations in the supply network. Confirm other gas appliances in the property are working normally as a quick sanity check.
How to fix it
- Reset the boiler once or twice DIY safe
Press and hold the reset button (usually marked with a flame symbol or the letter R) for about three seconds. If the boiler fires up and runs normally, monitor it closely over the next hour. A genuine one-off ignition glitch can occasionally clear this way. If F.27 returns within a short time, or keeps coming back over several days, further attempts at resetting without a repair are unlikely to help and could indicate a deeper fault.
- Check your gas supply DIY safe
Turn on a gas hob or check another gas appliance in the property. If nothing else is working from the gas supply either, contact your gas supplier rather than an engineer — there may be a network interruption or your meter may have tripped. Also confirm that the gas isolation valve on the boiler's gas service pipe (usually a quarter-turn lever) is fully open, with the lever running parallel to the pipe.
- Look for obvious signs of a water leak near the boiler DIY safe
Inspect the area beneath and immediately around the boiler for damp patches, drip marks, or corrosion staining on pipework. Do not touch internal components. If you find an active drip, turn off the mains water supply stop tap as a precaution to prevent water from spreading to electrical parts, then call an engineer. Also check the system pressure gauge — if it reads below 1 bar, a slow internal leak may be allowing moisture onto the electronics.
- Top up system pressure if it has dropped below 1 bar DIY safe
If your pressure gauge reads below 1 bar and there are no visible drips, you can repressurise via the filling loop (two small valves, usually under the boiler). Open both valves slowly, watch the gauge, and close them once the needle reaches 1.2–1.5 bar. Do not exceed 2 bar. If pressure keeps dropping back down, this points to a leak that needs professional investigation — do not keep topping up indefinitely.
- Do not attempt to access internal components Gas Safe engineer
The gas valve, PCB, flame sensor electrode, ignition leads, and burner assembly all sit inside the boiler casing. Removing the casing and working on gas-carrying components without Gas Safe registration is illegal under the Gas Safety (Installation and Use) Regulations 1998, and is dangerous. Leave all internal diagnosis and repair to a qualified engineer.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
An engineer experienced with Vaillant ecoTEC boilers will remove the casing safely, check for moisture ingress and dry or clean affected components, test the flame sensor and clean or replace the ionisation electrode, inspect the ignition leads and spark electrode for wear, and carry out a gas valve function test. If the PCB is implicated — particularly if multiple different fault codes have appeared — they will test board voltages and advise on replacement. Ask specifically for Vaillant ecoTEC experience, as misdiagnosis on this model can lead to unnecessary parts being fitted.
Parts you may need
- Vaillant ecoTEC PCB (main control board) · from £190
- Ionisation / flame sense electrode · from £25
- Ignition electrode and lead set · from £45
- Gas valve (Vaillant ecoTEC compatible) · from £120
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£350, depending on the underlying cause.
Frequently asked questions
Can I just keep resetting the boiler to clear the F.27 code?
Resetting once or twice is reasonable to rule out a one-off ignition hiccup, but repeatedly resetting a boiler showing F.27 is not a fix. The code indicates the boiler's safety system has detected something wrong with the flame signal, and continuing to override the lockout without investigation can mask a potentially serious fault. If F.27 comes back after one or two resets, book an engineer.
Why does my Vaillant ecoTEC keep switching between F.27, F.61, and F.62?
Cycling between different fault codes on successive resets is a classic symptom of a deteriorating PCB on the ecoTEC range. When the control board's internal components begin to fail — particularly the transformers — the voltages powering various circuits become unstable, causing the boiler to report different faults each time depending on which circuit misbehaves first. A Gas Safe engineer can test the board voltages to confirm this. PCB replacement typically costs around £190 for the part plus one to two hours of labour.
How much does it cost to fix a Vaillant F.27 fault?
Most F.27 repairs fall in the range of £120–£350 all-in, depending on what's causing it. Cleaning the flame sensor or replacing ignition components sits toward the lower end; replacing the gas valve or PCB pushes toward the higher end. If the PCB needs replacing on an older boiler, the part alone is around £190, and with labour the total can reach £350–£450 — at which point it's worth considering whether a new boiler makes more financial sense, especially on units over eight to ten years old.
Is the F.27 fault covered by my Vaillant warranty?
It depends. The standard Vaillant ecoTEC Plus warranty is five years (extendable to seven years if installed by an approved installer); the ecoTEC Pro typically carries two years (also extendable to seven). If the F.27 is caused by a manufacturing defect and your warranty is still active, Vaillant may cover the repair. However, faults caused by moisture ingress from a separate leak, lack of servicing, or general wear are usually excluded. Check your warranty documentation and contact Vaillant directly before booking an independent engineer if you think you may be covered.