How much does it cost to fix boiler error code C123?

Typical repair cost for the boiler C123 fault is £0-80 including labour.

C123 Low

Boiler Fault Code C123

Reviewed by the BoilerFaultCodes Editorial Team Last reviewed 27 April 2026

Fault code C123 appears across 1 boiler brand and 4 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning Your boiler is displaying a service notification indicating that a technician is attempting to enter or has entered the password-protected installer settings menu. (As reported on Atag A200)

Pick your boiler brand for the C123 fix

Atag C123 4 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 27 April 2026