Vaillant F27 Fault Code: Causes & How to Fix

Last Updated on February 10, 2025

If you own a Vaillant boiler and have encountered the F27 fault code, you’re not alone. This error indicates a flame detection issue, which can lead to your boiler shutting down for safety reasons. 

Understanding what causes the F27 fault code and how to fix it can help you restore your heating and hot water efficiently.

In this guide, we’ll cover:

• What the Vaillant F27 fault code means
• Common causes of the F27 error
• How to fix the issue
• When to call a Gas Safe engineer

Let’s dive in.

What Does the Vaillant F27 Fault Code Mean?

The F27 error code on a Vaillant boiler typically signals a flame detection problem. The boiler’s safety system detects an issue with the flame signal and shuts down the system to prevent potential gas leaks or unsafe combustion.

Common Causes of the Vaillant F27 Fault Code

Several factors can trigger the F27 fault code, including:

1. Faulty Flame Sensor

The flame sensor ensures the boiler is burning gas correctly. If it’s faulty or covered in debris, it might not detect a flame, triggering the F27 error.

2. Condensation or Moisture in the System

Water or condensation inside the boiler can interfere with electrical components, causing false flame detection readings. This often happens if the boiler is in a cold or damp location.

3. Defective PCB (Printed Circuit Board)

The PCB controls all boiler operations. If it develops a fault, it may incorrectly register a flame detection issue, leading to an F27 error.

4. Gas Valve Issues

If the gas valve is stuck or malfunctioning, it can lead to an improper gas-to-air ratio, causing the flame to behave unpredictably or fail to ignite.

5. Electrode & Ignition Problems

A faulty ignition electrode or spark generator can lead to failed ignition, causing the boiler to shut down and display the F27 fault code.

6. Flue or Air Intake Blockage

Restricted airflow due to a blocked flue or air intake can cause combustion issues, leading to incorrect flame signals and triggering the F27 fault.

How to Fix the Vaillant F27 Fault Code

Important: Some troubleshooting steps require the expertise of a Gas Safe registered engineer. If you’re not qualified, avoid opening the boiler casing.

1. Reset the Boiler

Sometimes, simply resetting the boiler can clear the fault if it was triggered by a temporary issue. To reset your Vaillant boiler:

1. Locate the reset button (usually marked with a flame symbol).
2. Hold the button for a few seconds.
3. Wait to see if the fault clears.

If the F27 error returns, further investigation is needed.

2. Check for Condensation or Moisture

If your boiler is installed in a cold or damp area, condensation might be affecting the electrical components. A professional can inspect and dry any affected parts.

3. Inspect the Flame Sensor & Electrodes

A Gas Safe engineer can:

• Clean or replace the flame sensor if it’s dirty or faulty.
• Check and replace faulty electrodes if needed.

4. Test the Gas Valve

A professional will:

• Ensure the gas valve is operating correctly.
• Adjust or replace it if it’s malfunctioning.

5. Examine the PCB (Printed Circuit Board)

A technician will:

• Test the PCB for faults.
• Replace it if necessary, as a faulty PCB can cause multiple boiler issues.

6. Check the Flue & Air Intake

Blocked or damaged flues can restrict airflow, leading to combustion problems. A Gas Safe engineer will inspect and clear any obstructions.

Other Common Vaillant Boiler Fault Codes

Many Vaillant boiler error codes share similar causes and troubleshooting steps. 

While the F27 fault code often relates to ignition issues caused by a faulty PCB or gas supply problems, other common errors—like the F28 and F29 codes—also indicate ignition failures but may be due to gas valve or supply issues.

If you’re experiencing recurring or multiple error codes, it’s helpful to understand how Vaillant fault codes work and what each one means:

• Vaillant F28 Fault Code – Another ignition fault, often caused by a failed gas valve or frozen condensate pipe. 
• Vaillant F29 Fault Code – Loss of flame during operation, potentially due to low gas pressure. 
• Vaillant F20 Fault Code – Overheating issue that could indicate a faulty pump or sensor. 

When to Call a Professional

If resetting the boiler doesn’t fix the issue, or if you’re unsure about performing any checks yourself, it’s essential to call a Gas Safe registered engineer.

Get a Free Boiler Quote – Upgrade to a More Reliable System

If your Vaillant boiler frequently displays error codes like F27, it may be time for an upgrade. Older boilers can become unreliable and inefficient, leading to higher energy bills and costly repairs. 

Take a look at our Vaillant boiler reviews and prices guide to discover the best models for your home and their costs.

Alternatively, get a FREE, no-obligation boiler quote today and enjoy:
✅ A more energy-efficient boiler.
✅ Lower heating bills.
✅ Reliable heating & hot water.
✅ Professional installation from trusted experts.

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Final Thoughts

While the F27 fault code is often caused by ignition failure or PCB issues, regular boiler maintenance can significantly reduce the risk of such faults occurring in the first place. A well-maintained boiler operates more efficiently, is less likely to suffer from ignition problems, and has a longer lifespan.

If your boiler hasn’t been serviced recently, this could be a contributing factor to the error. Regular servicing ensures that components like the ignition system, gas valves, and sensors are in good working order. 

But how often should a boiler be serviced to avoid unexpected breakdowns? Experts recommend an annual service to keep everything running smoothly and catch potential problems before they escalate. If you’re unsure what to expect from a service, understanding how long a boiler service takes can help you plan accordingly.