Vaillant F27 Fault Code: What It Means and How to Fix It

What does the Vaillant F27 fault code mean?

The Vaillant F27 fault code is a safety lockout that appears when the boiler detects a flame signal while the burner is supposed to be off. It usually points to a gas solenoid valve that isn't sealing, a faulty PCB, or a flame-detection electrode fault. This is not a DIY fix — it must be diagnosed by a Gas Safe registered engineer.

Vaillant's own meaning for F27 is that a flame signal has been detected while the burner is off. Engineers call this flame simulation: the boiler's monitoring electrode is reporting a flame when there shouldn't be one. Rather than trust a reading it can't verify, the boiler shuts itself down.

The lockout itself is protective. Your boiler is refusing to run on information it doesn't trust, which is exactly what you'd want it to do. It's safe to leave switched off while you arrange a repair.

Key facts

The quick version, before the detail.

• Meaning: a flame signal has been detected while the burner is off (Vaillant's official meaning).
• DIY-fixable? No — a single reset is the only safe homeowner step; otherwise it's engineer-required.
• Gas Safe job: yes — the likely causes are a gas solenoid valve, the PCB, or a flame-detection electrode.
• Vaillant warns: do not open the boiler casing.
• Smart Plan cover pays parts and labour up to £500 a year — but only up to £200 a year if the boiler is over 7 years old, which many boilers throwing an F27 will be. A major part such as a gas valve or PCB can cost more than the £200 cap, so cover reduces a bill rather than guaranteeing it is met in full.
• A £95 call-out fee applies (see the plan terms for when it's charged), and call-outs are Monday to Friday, 08:00 to 18:00 only — not evenings or weekends, unless it's a genuine emergency.
• Using the cover starts a 12-month minimum agreement. There's a 14-day cooling-off period before your first use; leaving early after that costs 75% of the remaining term.

Vaillant F27 at a glance

Why does the F27 code appear?

F27 points to one of three internal faults: a gas solenoid valve that isn't leaktight, a defective PCB (the boiler's control board), or a faulty flame-detection electrode. Any of these can make the boiler sense a flame that isn't actually there.

In real homes, moisture is a frequent trigger. Condensation forming on the electrical components, or water getting in from a leak elsewhere in the system, can confuse the flame-sensing circuit and bring on an F27.

That's also why F27 can keep coming back. Until the failing part is replaced — or the damp electronics are dried out and the source of the moisture fixed — a reset will only clear the code for a while before it returns.

Can I fix a Vaillant F27 myself?

No — F27 is not a DIY fix. The only safe thing you can try is a single reset, in case the lockout was a one-off glitch. If the code comes straight back, stop there.

Vaillant explicitly warns against opening the boiler casing, and with good reason: the gas solenoid valve, PCB and flame-detection electrode all sit inside the boiler, on the gas and electrical side. It's illegal for anyone who isn't Gas Safe registered to work on the gas components, and there's no homeowner check that reaches them safely.

What to do when you see F27

1. Reset the boiler once, following the steps in your Vaillant manual.

2. If F27 clears and doesn't return, keep an eye on it — it may have been a transient fault.

3. If F27 comes back, don't keep resetting. Book a Gas Safe registered engineer to diagnose it.

Is a Vaillant F27 fault dangerous?

The F27 lockout is the boiler protecting you, not a hazard in itself. It has shut down rather than operate on a flame reading it can't trust, so it's safe to leave switched off until an engineer can look at it.

Two things to avoid: don't keep resetting it — repeated resets only mask a genuine fault and can add wear — and don't open the casing. Book a Gas Safe registered engineer promptly instead.

F27 is not a gas emergency on its own. But if you ever smell gas or your carbon monoxide alarm sounds — whatever code is showing — leave the gas alone, open the windows, get everyone out, and call the National Gas Emergency Service on 0800 111 999.

When to call a Gas Safe registered engineer

Because every likely cause of F27 sits inside the boiler, it should go straight to a Gas Safe registered engineer if a single reset doesn't clear it. There's no further safe check to work through at home.

A gas valve or PCB is among the more involved boiler repairs, so it's worth having the fault diagnosed properly rather than guessing at the part. You can check any engineer is qualified on the Gas Safe Register before they start work.

You can book a one-off repair with us, or spread the cost of future faults with a Smart Plan boiler and central heating module. Before you set one up, here's the honest small print so you can weigh it up.

What a Smart Plan module actually covers

Smart Plan is a service plan, not insurance. A boiler and central heating module covers parts and labour up to your cover limit — up to £500 a year if your boiler is under 7 years old, or up to £200 a year if it's over 7 years old. Many boilers old enough to throw an F27 fall into that £200 tier, and a gas valve or PCB repair can cost more than £200, so the plan reduces a repair bill rather than guaranteeing it's paid in full.

A £95 call-out fee applies — check the plan terms for exactly when it's charged — and call-outs run Monday to Friday, 08:00 to 18:00 only, unless it's a genuine emergency, so a weekend or evening breakdown won't be attended as a routine visit. There's a 14-day cooling-off period, but once you use the cover a 12-month minimum agreement period begins, and leaving early after that costs 75% of the remaining term. Cover is from UK Boiler Company Ltd, trading since 2014.

Vaillant F27 fault FAQs

Seeing F27 on your Vaillant? Book a Gas Safe registered engineer.

Book a one-off repair and a Gas Safe registered engineer will diagnose the lockout. Or add a Smart Plan boiler and central heating module — a service plan, not insurance — that pays parts and labour up to your cover limit: up to £500 a year if the boiler is under 7 years old, or up to £200 if it's over 7. A £95 call-out fee applies, call-outs are Monday to Friday 08:00 to 18:00 unless it's a genuine emergency, and using the cover starts a 12-month minimum agreement (14-day cooling-off before first use).

Helpful links

• Vaillant F28 fault code
• Vaillant F29 flame-loss fault
• All Vaillant fault codes
• Boiler & central heating cover
• No heating? Book a repair
• Vaillant boiler fault codes (official)
• Check an engineer on the Gas Safe Register