S.23 High

Boiler Fault Code S.23

Also written as S23 — same fault.

Reviewed by the BoilerFaultCodes Editorial Team Last reviewed

Fault code S.23 appears across 3 boiler brands and 58 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning Your boiler is struggling to light because it is stuck in a cycle of checking for water flow before the ignition process starts. (As reported on Glow-worm Ultracom 2 SXI)

Pick your boiler brand for the S.23 fix

Glow-worm S.23 9 models Heatline S.23 8 models Vaillant S.23 41 models

Source

Information for the S.23 fault code is sourced from the official service / installation manual for each model that displays it. Read more about how we source fault codes →

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026