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 25 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.
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A cover plan would handle faults like S.23: no call-out fees, no surprise bills.
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Glow-worm models that show the S.23 fault code
Click a model for the full fault code list and model-specific repair notes.
- 30cxi Repair: £120-250
- 38cxi Repair: £120-250
- Betacom4
- Easicom3 Combi
- Easicom3 System
- Energy
- Energy Regular
- Energy System
- Energy7 Combi
- Energy7 Regular
- Energy7 System
- Essential
- Flexicom 24c / 28c
- Flexicom cx Repair: £100-£350
- Sustain 12s
- Ultimate 3 25R
- Ultimate3 Combi
- Ultimate3 System
- Ultracom 2 CXI Repair: £100-£350
- Ultracom 2 HXI Repair: £100-£350
- Ultracom 2 SXI Repair: £120-350
- Ultracom 24cx 30cx Repair: £100-£350
- Ultracom 35 Store Repair: £100-£350
- Ultracom cxi Repair: £100-£350
- Ultracom sxi Repair: £120-350
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 7 July 2026