8.4 High

Glow-worm 8.4 Fault Code

Also written as 84 — same fault.

The Glow-worm 8.4 fault code means: your boiler has detected that it is running too hot and has automatically switched itself off as a safety precaution to prevent damage.. 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: Overheat Cutoff Cuts Out Prematurely
Typical repair cost £150-300 (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 8.4 fault code

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

Source

Glow-worm 8.4 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