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

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

C123 Low

Atag C123 Fault Code

Q: What does the Atag C123 fault code mean?

Your boiler is displaying a service notification indicating that a technician is attempting to enter or has entered the password-protected installer settings menu.

The Atag C123 fault code means: your boiler is displaying a service notification indicating that a technician is attempting to enter or has entered the password-protected installer settings menu.. It appears on 4 Atag models in our database.

Technical description: Installer level access code

Typical repair cost £0-80 (including labour)

DIY fix possible

Press the 'Reset' or 'Back' button to return to the main displayMonitor the screen to see if the code clears and the heating continues as normalIf the code persists, restart the boiler using the power isolation switch

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Atag models that show the C123 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Atag C123 fault code information on this page is sourced and cross-checked against the official Atag service / installation manuals for each affected model.

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