Vaillant F27 Fault Code: Causes & What to Do

Vaillant F27 fault code? F27 means the boiler has detected a flame signal when the burner should be switched off.

Vaillant lists a gas solenoid valve that is not fully leak-tight and a defective PCB as possible causes. Professional fault-finding information also shows that the flame-detection system may need checking before any component is replaced.

This is not a fault to investigate by opening the boiler yourself. Record the code and arrange professional diagnosis if it returns.

What Does the Vaillant F27 Fault Code Mean?

Vaillant describes F27 as:

A flame signal has been detected while the burner is off.

In normal operation, the boiler should only detect a flame when the burner has been instructed to operate.

If the electronics receive a flame signal at the wrong point in the operating sequence, the boiler can shut down and display F27.

Some Vaillant technical manuals describe the condition as a flame simulation or a fault in flame detection.

This may leave you with:

• an F27 code on the display;
• no heating;
• no hot water on a combi boiler;
• a boiler that repeatedly locks out;
• a fault that returns after resetting.

Vaillant F27 Fault Code: What Should I Do First?

There are very few homeowner checks required for F27.

You can safely:

• write down the exact fault code;
• note the Vaillant boiler model;
• check whether heating and hot water have stopped;
• note whether the code returns after a normal manufacturer-approved reset;
• tell the engineer if any other fault codes have also appeared.

Do not remove the boiler casing or attempt to inspect the gas valve, electrodes, wiring or PCB.

3 Areas That May Cause a Vaillant F27 Fault

1. Gas Valve Is Not Fully Closing

Vaillant lists a gas solenoid valve that is not leak-tight as one possible cause of F27.

The gas valve controls the flow of gas to the burner. When the burner is switched off, the valve should close correctly.

If professional testing finds that the valve is allowing gas through when it should be closed, this can create an abnormal flame condition and trigger the fault.

This requires specialist testing.

Do not attempt to test, adjust or replace a boiler gas valve yourself.

A gas valve should also not be replaced purely because F27 appears on the display. It needs to be tested first.

2. PCB or Boiler Electronics Fault

The PCB is the boiler’s printed circuit board and helps control the burner, gas valve, ignition sequence and flame monitoring.

Vaillant lists a defective PCB as another possible cause of F27.

However, this does not mean F27 automatically requires an expensive new circuit board.

The gas valve, flame-detection system, electrical connections and operating sequence should be checked before the PCB is condemned.

If you want to understand this component in more detail, see:

Boiler PCB Faults: Signs, Causes & Diagnosis

3. Flame-Detection or Electrode Problem

Vaillant’s professional fault-finding guidance also refers to flame detection and the electrode when diagnosing recurring F27 faults.

The boiler uses its flame-detection system to confirm whether a flame is genuinely present.

If the electronics receive an incorrect flame signal at the wrong stage of operation, the fault may involve the flame-monitoring circuit rather than an actual continuing flame.

This is why a competent engineer needs to distinguish between:

• a genuine flame when the burner should be off;
• a gas valve allowing gas through;
• an incorrect flame-detection signal;
• a PCB fault;
• another electrical problem.

Diagnostic point: the F27 code describes what the boiler has detected. It does not prove which component has failed.

Does Vaillant F27 Mean There Is a Gas Leak?

Not necessarily.

F27 is a boiler fault relating to an unexpected flame signal. It does not automatically mean gas is leaking into your home.

However, because Vaillant lists a gas valve that is not leak-tight as one possible cause, the boiler should be professionally checked rather than repeatedly reset.

If you actually smell gas or suspect a gas escape, treat that as a separate gas emergency.

In Great Britain, call the National Gas Emergency Service on 0800 111 999.

Can I Reset a Vaillant F27 Fault?

If the user manual for your particular Vaillant boiler allows a normal reset, you can follow that procedure.

Write down the F27 code before resetting it.

A reset may temporarily restore operation, but it does not repair a gas-valve, flame-detection or PCB fault.

If F27 returns, stop repeatedly resetting the boiler and arrange fault diagnosis.

If your boiler continually starts and then locks out again, also see:

Boiler Keeps Turning Off? Causes & What to Do

What Is the Difference Between Vaillant F27 and F28?

These two faults both involve flame operation, but they describe very different conditions.

• F27: a flame signal is detected when the burner should be off.
• F28: a flame signal is not detected when the boiler is trying to ignite.

So F27 relates to an unexpected flame signal, while F28 relates to unsuccessful ignition.

If your boiler actually shows F28, see:

Vaillant F28 Fault Code: Causes & What to Do

What Is the Difference Between Vaillant F27 and F29?

F29 is another flame-related Vaillant fault, but again it means something different.

• F27: flame signal detected while the burner should be off.
• F29: the flame was lost during operation and the boiler could not successfully reignite.

F29 can therefore be connected with issues such as gas supply, flue-gas recirculation, blocked condensate drainage or ignition problems.

The exact code shown on the display is important when diagnosing the boiler.

What Is the Difference Between Vaillant F27 and F62?

F27 and F62 can both involve the gas valve or boiler electronics, but they identify different detected conditions.

F27 means the boiler has detected a flame signal while the burner is supposed to be off.

F62 means flame switch-off has been detected after a delay.

Vaillant lists the gas valve and PCB among possible causes of both faults, but the diagnostic sequence is not identical.

Do not assume that because the same component appears on two fault-code lists, it definitely needs replacing.

Can a Faulty PCB Cause F27?

Yes. Vaillant officially lists a defective PCB as one possible cause.

The PCB processes flame information and controls other components involved in combustion.

However, replacing a PCB without first checking the gas valve and flame-detection system can be an expensive guess.

A proper diagnosis should establish whether the incorrect signal originates from the PCB or from another component.

Can a Faulty Gas Valve Cause F27?

Yes.

Vaillant lists a gas solenoid valve that is not leak-tight as one of the primary possible causes of F27.

Vaillant’s professional guidance recommends that an engineer carry out the appropriate gas-valve tightness checks before deciding whether it needs replacement.

This is gas work and is not a homeowner test.

Can an Ignition Electrode Cause F27?

The ignition and flame-detection system may form part of the diagnostic process.

Vaillant’s professional guidance notes that if F27 continues to appear at the wrong stage of the lighting sequence after other checks, the PCB or electrode may require further investigation.

Again, this should be confirmed through testing rather than replacing the electrode based on the fault code alone.

Can Low Boiler Pressure Cause F27?

F27 is not a low-water-pressure fault.

Vaillant’s main low-pressure fault is F22.

If your boiler pressure is also low, that may be a separate issue that should be reported alongside the F27 code.

For recurring pressure loss, see:

Why Does My Boiler Pressure Keep Dropping?

Can I Fix a Vaillant F27 Fault Myself?

No internal F27 repair should be attempted by a homeowner.

You can:

• record the fault code;
• record the boiler model;
• note when the fault appears;
• follow the normal user reset procedure if your manual allows it;
• report whether heating and hot water are affected.

You should not:

• remove the boiler casing;
• test the gas valve;
• adjust gas settings;
• remove or clean internal electrodes;
• test live electrical components;
• replace the PCB yourself.

How Is a Vaillant F27 Fault Diagnosed?

An engineer should confirm what is causing the unexpected flame signal before replacing components.

Depending on the boiler model and symptoms, diagnosis may involve checking:

• whether a genuine flame is present when the burner should be off;
• gas-valve operation and tightness;
• flame detection;
• electrode condition and operation;
• electrical connections;
• the PCB;
• the boiler operating sequence;
• manufacturer-specific technical information.

Engineer point: an F27 code should lead to testing, not automatic replacement of the gas valve or PCB.

Why Does Vaillant F27 Keep Coming Back?

If F27 disappears after resetting but later returns, the underlying fault may still be present.

When booking an engineer, note:

• whether the boiler runs after being reset;
• how long it operates before F27 returns;
• whether it happens during heating or hot-water demand;
• whether any other fault code appears first;
• whether heating and hot water both stop.

This information can help establish where in the operating sequence the fault occurs.

How Much Does It Cost to Fix Vaillant F27?

There is no useful single F27 repair price because the cost depends on what diagnosis finds.

A repair could involve:

• the gas valve;
• flame-detection components;
• an electrode;
• electrical connections;
• the PCB;
• another model-specific fault.

The parts and labour involved differ significantly between these possibilities.

For this reason, the boiler should be tested first and the confirmed repair priced afterwards.

Does F27 Mean I Need a New Vaillant Boiler?

No.

An F27 fault alone does not mean the appliance needs replacing.

A confirmed gas-valve, PCB or flame-detection fault may be repairable.

Replacement becomes worth comparing when the appliance is older, has several other significant faults or the overall condition makes another major repair difficult to justify.

If you are comparing the options, see:

New Boiler Cost Guide

When Should I Call a Boiler Engineer?

Arrange professional diagnosis if:

• F27 appears on the boiler;
• F27 returns after a normal reset;
• the boiler repeatedly locks out;
• heating or hot water is unavailable;
• another flame-related fault code also appears;
• you have been told the gas valve or PCB may be faulty.

For professional boiler fault diagnosis, visit:

Boiler Repairs

What Should I Do If I Smell Gas?

F27 itself does not automatically mean there is a gas leak in your property.

However, if you smell gas or suspect a gas escape, do not continue resetting or operating the appliance.

In Great Britain, call the National Gas Emergency Service immediately on 0800 111 999.

Vaillant F27 Fault in Cardiff?

If you’re in Cardiff and your Vaillant boiler is displaying F27, repeatedly locking out or has a suspected flame-detection, gas-valve or PCB fault, visit:

Boiler Repair in Cardiff

When booking, tell us:

• your Vaillant boiler model;
• the exact F27 code;
• whether the fault returns after resetting;
• whether heating and hot water both stop;
• whether any other fault codes appear.

Quick Questions

What does F27 mean on a Vaillant boiler?

F27 means the boiler has detected a flame signal when the burner should be switched off.

What causes Vaillant F27?

Vaillant lists a gas solenoid valve that is not leak-tight and a defective PCB as possible causes. The flame-detection system may also need checking during professional diagnosis.

Does F27 mean the gas valve is definitely faulty?

No. A gas-valve problem is one possible cause, but the PCB and flame-detection system can also be involved. The valve should be professionally tested before it is replaced.

Can a faulty PCB cause Vaillant F27?

Yes. Vaillant officially lists a defective PCB as one possible cause, but the other relevant components should be tested before replacing it.

Can I reset a Vaillant F27 fault?

If your Vaillant user instructions allow a normal reset, you can follow that procedure. If F27 returns, arrange professional diagnosis instead of repeatedly resetting the appliance.

Is Vaillant F27 dangerous?

F27 involves abnormal flame detection and can be associated with the gas valve or boiler electronics. Do not remove the casing or bypass the fault. Have a recurring F27 fault professionally investigated.

Does F27 mean there is a gas leak?

Not automatically. F27 is an appliance fault code. If you can actually smell gas or suspect a gas escape, call the National Gas Emergency Service on 0800 111 999.

Need Help With a Vaillant F27 Fault?

Let’s Heat can diagnose Vaillant F27 faults, recurring boiler lockouts, flame-detection problems and suspected gas-valve or PCB faults.

You do not need to identify the failed component yourself before booking.

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Sources Checked

This guide has been checked against current Vaillant fault-code information, Vaillant professional fault-finding guidance and HSE gas-safety guidance.

Vaillant – Official Boiler Fault Codes

Vaillant Professional – Fault-Finding Guidance

HSE – Gas Safety Guidance