BoilerFaultCodesGlow-worm F3 Fault Code
The Glow-worm F3 fault code means: your boiler has switched itself off because it has detected it is getting too hot, likely due to a blockage or a pump that isn't circulating water properly.. It appears on 5 Glow-worm models in our database.
Technical description: Heat exchanger blockage or pump fault (Overheat)
What causes the Glow-worm F3 fault?
This fault occurs because the internal sensors have detected that the water inside the boiler is reaching a dangerously high temperature. It is usually caused by a pump failure that prevents water from moving away from the burner, or a build-up of debris that has physically blocked the heat exchanger. Because the heat cannot be carried away to your radiators or taps, the boiler shuts down to prevent internal damage.
This issue often surfaces when you first turn your heating back on in autumn, as pumps can seize up after being idle during the summer months.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Glow-worm models that show the F3 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm F3 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