S.99 Low

Glow-worm S.99 Fault Code

Also written as S99 — same fault.

The Glow-worm S.99 fault code means: your boiler is currently refilling its internal water pressure system automatically and will return to normal operation once the process is complete.. It appears on 16 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: Automaticfillingactive

DIY fix possible

Wait for approximately 10 to 15 minutes for the automatic filling process to finish.Do not turn off or reset the boiler while this status code is displayed.If the code persists for more than an hour, check if your external filling loop valves are fully closed.

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Glow-worm models that show the S.99 fault code

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

Source

Glow-worm S.99 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