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The Glow-worm F28 fault code means your boiler has failed to ignite. In many cases, the boiler has attempted to light several times before locking out and displaying the error.

Sometimes the cause is simple, such as a temporary interruption to the gas supply. In other cases, the fault is linked to the ignition system, gas valve, PCB or wiring.

This guide explains what the F28 fault means, the most common causes and which checks are safe to carry out before contacting a Gas Safe engineer. If you're seeing a different error, our guide to Glow-worm boiler fault codes explains what the most common fault codes mean.

How-to-Fix-the-F28-Glow-Worm-Boiler-Fault-Code
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What Does the Glow-worm F28 Fault Code Mean?

The Glow-worm F28 fault code is an ignition fault.

The boiler cannot establish a flame during the ignition sequence, so it locks out to protect the appliance. The problem may be caused by a lack of gas, a failed ignition component or an electrical fault within the boiler.

Common Causes of a Glow-worm F28 Fault

The most common causes include:

  • Gas supply problems
  • Frozen or blocked condensate pipe
  • Blocked flue
  • Faulty gas valve
  • PCB fault
  • Worn ignition electrode
  • Faulty spark generator
  • Burner debris
  • Sensor faults
  • Damaged wiring

Each of these faults prevents the boiler from completing its normal ignition sequence.

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How to Fix a Glow-worm F28 Fault

Some basic checks are safe for homeowners.

Check the gas supply

Check whether other gas appliances in the property are working.

If nothing has gas, the issue may be with your supply rather than the boiler.

Reset the boiler

Use the manufacturer's reset procedure.

If the F28 fault immediately returns after one reset, avoid repeatedly resetting the appliance.

Look for obvious problems

If cold weather has frozen the condensate pipe, thawing it may clear the fault.

Only carry out visual checks. Internal boiler repairs should always be left to a Gas Safe registered engineer.

Can You Fix an F28 Fault Yourself?

Usually, no.

Apart from checking your gas supply, resetting the boiler and thawing a frozen condensate pipe, diagnosing an F28 fault normally requires specialist testing.

Many of the possible causes involve gas components or live electrical parts that should only be worked on by a qualified engineer.

When to Call a Gas Safe Engineer

Arrange a repair if:

  • the boiler still shows F28 after resetting
  • the gas supply is working normally
  • the boiler repeatedly locks out
  • you suspect a gas valve, PCB or ignition fault

A heating engineer can safely test each component and identify the underlying fault without replacing parts unnecessarily.

Need Help with a Glow-worm F28 Fault?

Our Gas Safe registered engineers diagnose Glow-worm ignition faults every day.

Whether the problem is a faulty gas valve, ignition electrode, PCB or another component, we can identify the cause and recommend the most cost-effective repair. If the boiler still displays the F28 fault after these checks, our boiler repair in London service can diagnose and repair the problem safely.

If your boiler is no longer economical to repair, we'll also advise whether replacement is the better long-term option.

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