Vaillant F27 Fault Code: What It Means and What to Do

F.27 on a Vaillant boiler means the boiler has detected a flame signal while the burner is meant to be off. Vaillant's fault code list gives two causes: a gas solenoid valve that is not leaktight, and a defective printed circuit board. You can reset the boiler once, and it may run for a while, but neither cause fixes itself. If F.27 comes back, turn the boiler off and book a Gas Safe engineer. If you can smell gas at any point, don't reset anything: leave the property and call 0800 111 999.

What F27 means

Every modern gas boiler watches its own flame. A sensing electrode sits in the burner, and the circuit board reads a small electrical signal from it. When there is a flame, there is a signal. When the gas valve closes and the flame goes out, the signal should drop to nothing.

F.27 is what the boiler shows when that logic doesn't add up: the burner has been told to stop, but the board still reads a flame signal. Engineers often call it a "false flame" or "flame simulation" fault. The boiler can't tell whether there really is a flame burning where it shouldn't be, or whether the sensing circuit is lying to it, so it locks out and waits for someone to find out which.

Why it matters

Most Vaillant fault codes are about water: low pressure (F.22), poor circulation (F.75, F.83) or a blocked condensate pipe stopping ignition (F.28). F.27 is different because one of its causes is on the gas side. A gas valve that doesn't seal fully can let a small amount of gas through when it should be closed, keeping a flame alive or building up gas in the combustion chamber.

That's why this is a code to take seriously even if the boiler seems to be working again after a reset. The shutdown is a safety feature doing its job.

What you can safely do

There's very little a homeowner can do about F.27 itself, but a few quick observations make the engineer's visit faster:

1. Note exactly what the display says, and take a photo. F.27, F.28 and F.29 are easy to mix up at a glance, and the difference matters.
2. Reset once. Use the reset button as your user manual describes. On most ecoTEC models it's a single press. If the boiler fires and runs normally, keep an eye on it. If F.27 returns straight away or within a few days, stop resetting.
3. Look for water. Check under the boiler and on the casing for drips or damp. Moisture reaching the electrics is a known trigger for flame-sensing faults. If you can see water, turn the boiler off and read our guide to a boiler leaking water.
4. Check the condensate pipe in cold weather. If the white plastic pipe outside is frozen, condensate can back up into the boiler. Vaillant links that mainly to F.28, but water where it shouldn't be can confuse other readings too.
5. Leave the casing on. The gas valve, electrode and PCB are all behind the front panel. Only a Gas Safe registered engineer should work there.

If you're not sure how to reset your model, our general guide on how to reset a boiler explains what a reset does and doesn't do.

Likely causes

The first two are the causes Vaillant lists. The others are what engineers commonly find when they investigate, and they explain why an F.27 sometimes appears after a leak or a very wet spell. Diagnosing which one it is needs test equipment and access inside the sealed combustion area.

F27 vs F28 and F29

These three codes are all about the flame, but they point in different directions:

• F.27: a flame signal when the burner should be off. Gas valve or PCB. Engineer job.
• F.28: no flame detected during ignition. Often a gas supply problem, an empty prepayment meter or a frozen condensate pipe, so there are household checks worth doing first. See our Vaillant F28 guide.
• F.29: the flame went out while the boiler was running. See our Vaillant F29 guide.

For a wider view of Vaillant and other brands' codes, our boiler error codes guide explains how to read the manufacturer's list for your model.

What the engineer will do

A Gas Safe engineer will usually:

• Read the fault history on the boiler, which shows how often F.27 has occurred and what else was logged alongside it.
• Check whether the gas valve closes fully, and test the flame signal with the burner on and off.
• Inspect the electrode, its lead and the board for damp, corrosion or damage, and trace any water to its source.
• Replace the faulty part, then carry out combustion and gas tightness checks before handing the boiler back.

Always check the engineer's Gas Safe ID card before they start. Our page on Gas Safe engineers explains how to verify registration. If your boiler is still under Vaillant's warranty, contact Vaillant first, as outside work can affect the warranty.

We handle Vaillant faults as part of our boiler repair service. Our labour is £95 an hour, the £60 call-out is waived on jobs over 2 hours, and we confirm the price, including any parts, before work starts.

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