Vaillant F27 fault code: what it means and what to do

What does F27 mean on a Vaillant boiler?

F27 (shown as F.27) is Vaillant's code for "safety switch-off: flame simulation". In Vaillant's own wording, the monitoring electrode is reporting an incorrect flame — the boiler's flame-detection system is registering a flame signal at a moment when there should be none, typically when the gas valve is closed and the burner is meant to be off.

Vaillant boilers detect the burner flame using an ionisation (flame-sensing) electrode. A real flame conducts a tiny electrical current the control board can read. If the board sees that "flame present" signal when it has commanded the gas off, it cannot be sure the gas supply is actually shut.

Rather than risk unburnt gas, it shuts down and displays F27. This is a deliberate safety lockout, not the boiler "breaking" at random.

The wording is consistent across the Vaillant ecoTEC range (ecoTEC plus, ecoTEC pro and similar combi and system models), so the meaning of F27 does not change meaningfully from one ecoTEC model to the next.

Common causes of F27

F27 points at the flame-sensing chain rather than one single part. Typical causes a Vaillant engineer will be looking at include:

• Moisture or condensation on the electronics — damp on the PCB or ignition leads is one of the most common triggers and can cause a stray flame signal.
• A gas valve not closing fully — if the valve seeps, a residual flame can linger and be read as a false signal.
• A faulty or contaminated flame-sensing (ionisation) electrode or ignition leads — worn, dirty or cracked leads can give a misleading reading.
• A faulty PCB / flame monitor — in many real-world cases F27 is traced back to the control board misreading the ionisation circuit.

Because several of these can produce the same code, F27 needs proper diagnosis with the right tools — guessing at the part is how money gets wasted.

What you can safely check

F27 is an engineer-only fault, so there are no DIY repair steps. There are only a few universal, safe checks before you call out an engineer:

1. Check there is no gas smell. If there is, stop — do not reset — leave the property and call 0800 111 999.
2. Make sure the gas supply is actually on (other gas appliances working is a good sign), and that the boiler has power.
3. Try one reset using the boiler's reset button — see how to reset your boiler. If F27 clears and stays clear, good. If it returns, leave the boiler off. Do not keep resetting — repeatedly clearing a safety lockout is unsafe.
4. Glance at the pressure gauge — F27 is not a pressure fault, but it is worth knowing your reading (what your boiler pressure should be) before the engineer arrives.

That is the limit of safe homeowner action for F27. Do not open the boiler casing, and never touch the gas valve, burner, flue, ionisation electrode or PCB.

When to call a Gas Safe registered engineer

Call an engineer whenever F27 returns after a single reset, or immediately if you have any concern about gas. By law, only a Gas Safe Register engineer may work on the gas supply, gas valve, burner, flue, sealed combustion circuit or PCB. It is illegal and dangerous for anyone unregistered to attempt this work.

A competent engineer will usually:

• Inspect for moisture on the PCB, ignition leads and connectors, and dry or replace as needed.
• Check and clean or replace the flame-sensing (ionisation) electrode and ignition leads.
• Test the gas valve for correct shut-off and the ionisation signal at the board.
• Replace the PCB / gas valve where diagnosis confirms it, and run a full combustion and safety check afterwards.

An F27 lockout is a form of boiler lockout, and a seeping or sticking valve may also be flagged as a faulty gas valve during diagnosis.

Typical Vaillant F27 repair cost

Costs vary by region, model and whether a part is cleaned or replaced. These are indicative 2026 UK ranges; your own quote should come from the attending engineer. See also our wider guide to boiler repair costs.

Work	Indicative 2026 cost (£)
Diagnostic / call-out	£70 – £120
Flame sensor / ionisation electrode (fitted)	£90 – £180
Ignition leads (fitted)	£90 – £180
Gas valve repair or replacement (fitted)	£200 – £400
PCB (control board) replacement (fitted)	£300 – £600

Many F27 call-outs are resolved simply by drying out moisture or cleaning the electrode, which keeps the bill close to the diagnostic charge. A confirmed PCB or gas valve replacement is where costs climb.

Related Vaillant codes

If you are working through your boiler's display, these sibling guides may help:

• Vaillant F28 — ignition failure (no flame established at start-up).
• Vaillant F29 — flame lost during operation.
• Vaillant F22 — low water pressure / dry-fire fault.

For the full list, see our Vaillant fault codes hub.

Will boiler cover pay for an F27 repair?

This is general information, not advice — always check your own policy wording.

A flame-detection fault on a working boiler is the kind of breakdown many boiler-cover and home-emergency policies are designed for, and a policy that includes parts and labour would typically cover the engineer visit and a gas valve, electrode or PCB, subject to any excess.

Whether yours pays out depends entirely on your specific terms.

The usual caveats apply: most policies exclude pre-existing faults, may not pay if the boiler is beyond economical repair or has not been serviced, and often have an initial waiting period. Check exactly what is included in what boiler cover includes, and weigh it up against a single repair bill in is boiler cover worth it.

This article is general information, not gas-safety or financial advice. Always have gas appliances checked and repaired by a Gas Safe registered engineer. In a gas emergency, call the National Gas Emergency line on 0800 111 999. Costs are indicative UK guides for 2026.