Low
S.99 on Glow-worm Sustain 12s

Your boiler is currently performing a brief automatic self-check and should return to normal operation shortly.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

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.

What the manual says

Manufacturer manual

Internal automatic test programmes

From this model's manual
Verified against the Glow Worm Sustain 12s installation & service manual, page 33.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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