Protherm F.27 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.27 fault code mean?
The F.27 code means your Protherm boiler has detected a flame signal at a point when it shouldn't exist — or the flame supervision circuit has reported an unexpected flame reading during the ignition sequence. Protherm shares its error code architecture with Vaillant (both are part of the same group), so F.27 carries identical meaning across both brands. In practical terms, the boiler's control board opened the gas valve and attempted to light the burner, but the flame-sensing circuit reported something inconsistent — either a flame was detected before ignition was attempted, or the signal from the sensing electrode was unreliable or out of sequence. Because a rogue flame signal can indicate a dangerous condition (such as a gas valve that isn't closing fully), the boiler immediately locks out as a safety measure, cutting both central heating and hot water until the fault is cleared and the root cause investigated.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. 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 or condensation on electrical components Common
This is by far the most frequent trigger for F.27 on Protherm boilers. Condensate from the plastic sump or trap can drip onto the PCB or flame-sensing wiring below. Even a small amount of moisture on the electrode connections or board tracks can produce a spurious flame signal. Boilers over eight years old are particularly prone to this, as seals on the condensate trap degrade over time. Check underneath the boiler for any signs of drips or watermarks on components.
- Faulty or dirty flame-sensing electrode Common
The flame-sensing probe uses a small electrical current passing through the burner flame to confirm ignition. If the ceramic insulator on the electrode has cracked, if the probe tip is coated in soot or carbon deposits, or if the probe has simply worn out, it can give erratic or false readings — triggering F.27. This is a relatively inexpensive fix when identified early.
- Worn or damaged ignition leads and electrodes Sometimes
The high-voltage ignition leads that deliver the spark to the burner can develop cracks in their insulation over time, causing leakage to earth. This can interfere with the flame-sensing circuit (which shares the same electrode assembly on many Protherm models), producing false signals and locking out the boiler with F.27.
- Gas valve fault Sometimes
The gas valve controls gas flow to the burner. If the valve is sticking open slightly, is slow to close, or has an internal fault affecting its position feedback, the boiler may detect gas reaching the burner at the wrong moment in the ignition sequence — registering as a false flame. Gas valve issues are also associated with incorrect gas-to-flame ratio problems that can cause F.27.
- PCB (printed circuit board) failure Rare
The PCB interprets the tiny electrical signal from the flame-sensing electrode and decides whether a flame is genuinely present. If the board itself has developed a fault — often due to age, corrosion, or moisture ingress — it may misread the signal and generate F.27 spuriously. PCB failure tends to be a diagnosis of exclusion once other causes have been ruled out, and it is more common on boilers over ten years old.
How to fix it
- Reset the boiler once or twice and observe whether the fault returns DIY safe
Press and hold the reset button on your Protherm boiler as described in your user manual (typically three seconds). If F.27 clears and the boiler runs normally, monitor it over the next hour. A one-off lockout after a power interruption or cold snap can sometimes resolve itself on a reset. However, if F.27 returns within a short period, do not keep resetting — repeated lockouts indicate an underlying fault that needs investigation.
- Check that your gas supply is working DIY safe
Confirm that other gas appliances in your home — your hob or gas fire — are working normally. If they are not, the issue may be with your gas supply rather than the boiler. Contact your gas supplier or call the National Gas Emergency line on 0800 111 999 if you suspect a supply problem or smell gas. Never touch the boiler's gas valve yourself.
- Inspect underneath the boiler for condensate drips DIY safe
Look at the white plastic condensate trap and sump beneath your boiler for any signs of dripping water or watermarks on the components below. A leaking condensate trap is a common cause of moisture reaching the PCB or electrode wiring. If you can see an active drip, note its location and report this to your engineer — it will help speed up diagnosis.
- Do not attempt to touch the flame sensor, gas valve, ignition leads, or PCB Gas Safe engineer
All components associated with F.27 — the flame-sensing electrode, ignition leads, gas valve, and PCB — must only be inspected, tested, or replaced by a Gas Safe registered engineer. Working on gas components without registration is illegal in the UK and potentially dangerous. If there is any possibility the gas valve is not closing fully and you smell gas, evacuate immediately and call 0800 111 999.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
An engineer will use a flue gas analyser and test equipment to check the flame-sensing electrode signal strength, inspect the electrode and ceramic insulator for cracks or carbon fouling, test the ignition lead insulation, check the gas valve for correct operation and closure, and assess the PCB. Once the root cause is identified they will clean, repair, or replace the faulty component. Always ask to see the engineer's Gas Safe ID card before work begins.
Parts you may need
- Flame sensing electrode · from £35
- Ignition electrode and lead set · from £55
- Condensate trap / sump assembly · from £30
- Gas valve · from £130
- PCB (printed circuit board) · from £220
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–£380, depending on the underlying cause.
Frequently asked questions
Is the Protherm F.27 the same as the Vaillant F.27?
Yes — Protherm is owned by the Vaillant Group and the two brands share the same control system architecture. F.27 means exactly the same thing on both: the boiler has detected a flame signal that is out of sequence or inconsistent, causing a safety lockout. Diagnostic and repair procedures are essentially identical across Protherm and Vaillant models carrying this fault.
Can I fix F.27 myself, or do I need a Gas Safe engineer?
The homeowner checks you can safely do yourself are limited to resetting the boiler (once or twice), confirming your gas supply is on, and looking for condensate drips underneath the boiler. Everything beyond that — testing the flame sensor, inspecting the gas valve, and any component replacement — must be carried out by a Gas Safe registered engineer. This is a legal requirement in the UK, not just a recommendation.
How much does it typically cost to fix a Protherm F.27 fault?
For the most common repairs — cleaning or replacing the flame-sensing electrode, renewing ignition leads, or fixing a condensate leak — most homeowners pay between £120 and £380 including labour. A gas valve replacement sits toward the higher end of that range. A PCB replacement is more expensive, often £400–£600 all-in, and if your boiler is over ten years old and needs a new PCB or gas valve, it is worth getting a replacement boiler quote at the same time, as a new A-rated boiler may work out better value over the long run.
Why does my Protherm keep showing F.27 after I reset it?
If F.27 returns repeatedly after resetting, the underlying fault has not gone away. The most likely culprits are a moisture-damaged PCB or electrode, a cracked flame-sensing probe, or a gas valve that is not operating correctly. Stop resetting and call an engineer — repeated lockouts can occasionally make underlying faults harder to diagnose, and in the case of a sticking gas valve, continued operation could be unsafe.