BoilerFaultCodesGlow-worm S.23 Fault Code
Also written as S23 — same fault.
The Glow-worm S.23 fault code means: your boiler is struggling to light because it is stuck in a cycle of checking for water flow before the ignition process starts.. It appears on 9 Glow-worm models in our database.
Technical description: ignition
What causes the Glow-worm S.23 fault?
This fault occurs when the boiler's internal computer detects that water isn't moving through the system quickly enough to safely start the burner. It is usually caused by a failing circulation pump that can't push the water round, or a blocked heat exchanger that is restricting the flow. Essentially, the boiler stays in a holding pattern to prevent itself from overheating during the ignition phase.
This issue often surfaces in autumn when the heating is turned back on after a long summer break, as pump components can seize up during periods of inactivity.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Glow-worm models that show the S.23 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm S.23 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