C123 Low

Boiler Fault Code C123

Reviewed by the BoilerFaultCodes Editorial Team Last reviewed

Fault code C123 appears across 1 boiler brand and 3 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning Your boiler is currently locked in an installer-only settings menu and requires a specific access code to exit this mode and return to normal operation. (As reported on Atag Q 25S)

Pick your boiler brand for the C123 fix

Atag C123 3 models

Source

Information for the C123 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 7 July 2026