BoilerFaultCodesGlow-worm S.31 Fault Code
Also written as S31 — same fault.
The Glow-worm S.31 fault code means: your boiler is currently in summer mode, which means it will provide hot water but has temporarily disabled your central heating.. It appears on 12 Glow-worm models in our database.
Technical description: summer mode
What causes the Glow-worm S.31 fault?
This status occurs because the heating control or the thermostat has been set to provide hot water only, effectively turning off the central heating circuit. It is often triggered manually by a user switching to 'summer mode' or automatically when an outdoor sensor detects that the weather is warm enough to no longer require home heating. There is no mechanical failure; the boiler is simply following instructions to save energy by ignoring heating demands.
This code is most common during late spring and summer when homeowners intentionally disable their heating or when outside temperatures rise.
DIY fix possible
Locate the 'Mode' button or the heating temperature dial on the boiler control panel.Increase the central heating temperature setting to a level above the minimum (usually indicated by a radiator icon).The 'S.31' code should disappear and be replaced by a standard heating or hot water status code.
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Gas Safe Register →Glow-worm models that show the S.31 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm S.31 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