S.40 Low

Glow-worm S.40 Fault Code

Also written as S40 — same fault.

The Glow-worm S.40 fault code means: your boiler is simply indicating that it has successfully received a signal to turn on the central heating and is preparing to warm your home.. It appears on 2 Glow-worm models in our database.

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description: Heating mode activated

DIY fix possible

This fault may be fixable without an engineer. Check your manual first.

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Glow-worm models that show the S.40 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Glow-worm S.40 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 31 August 2026