BoilerFaultCodesGlow-worm S.04 Fault Code
Also written as S04 — same fault.
The Glow-worm S.04 fault code means: your boiler is operating normally and is currently heating up to provide warmth or hot water.. It appears on 12 Glow-worm models in our database.
Technical description: burner on
What causes the Glow-worm S.04 fault?
This is actually a normal status message rather than a fault, appearing when the boiler has successfully ignited the burner to meet a demand for heating or hot water. It indicates that the internal components have completed their safety checks and the boiler is now actively burning gas to heat your home. You do not need to take any action, as this code simply confirms the system is working as intended.
You will see this code more frequently during the winter months when your heating is running regularly to keep your home warm.
DIY fix possible
No action is required as this is an status update rather than a fault.Check your thermostat settings if you did not expect the heating to be on.Monitor the display to see if it changes once the home reaches the desired temperature.
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Gas Safe Register →Glow-worm models that show the S.04 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm S.04 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