BoilerFaultCodesGlow-worm F05 Fault Code
The Glow-worm F05 fault code means: your boiler has switched itself off because it has gotten too hot, likely due to a circulation issue or a component failure.. It appears on 1 Glow-worm model in our database.
Technical description: Overheat fault - Safety thermostat has activated
What causes the Glow-worm F05 fault?
This fault occurs when the water inside the boiler reaches an unsafe temperature, triggering a safety sensor to shut the system down to prevent damage. The most common reasons are a failed pump that isn't moving water away from the heat exchanger quickly enough, or a blockage caused by a build-up of debris and sludge within the system.
This fault is more frequent when you first turn the heating back on for autumn, as pumps can seize up after being idle throughout the summer.
DIY fix possible
Check that all radiator valves are fully openCheck that the boiler pressure is between 1.0 and 1.5 bar on the gaugeWait 30 minutes for the unit to cool down and try pressing the reset button
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Gas Safe Register →Glow-worm models that show the F05 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm F05 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