BoilerFaultCodesGlow-worm F.25 Fault Code
Also written as F25 — same fault.
The Glow-worm F.25 fault code means: your boiler has shut down because it detected that the exhaust gases are getting too hot, which is a safety feature to prevent internal damage.. It appears on 6 Glow-worm models in our database.
Technical description: Safety switch-off – flue gas temperature too high
What causes the Glow-worm F.25 fault?
This fault occurs when the internal safety sensors detect that the waste gases leaving your boiler have reached a dangerously high temperature. The most common reason is a buildup of debris or limescale inside the main heat exchanger, which prevents heat from being transferred to your water and instead sends it out through the flue. It can also be caused by a faulty sensor sending incorrect readings to the boiler's computer.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Glow-worm models that show the F.25 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm F.25 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 27 April 2026