F.4 High

Glow-worm F.4 Fault Code

Also written as F4 — same fault.

The Glow-worm F.4 fault code means: your boiler is successfully igniting but is unable to keep the flame burning, causing it to shut down shortly after starting.. It appears on 1 Glow-worm model in our database.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description: Flame goes out whilst lit during a demand
Typical repair cost £150-350 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Glow-worm models that show the F.4 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Glow-worm F.4 fault code information on this page is sourced and cross-checked against the official Glow-worm service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026