C123 Low

Atag C123 Fault Code

The Atag C123 fault code means: 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.. It appears on 3 Atag models in our database.

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description: Installer access code

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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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 7 July 2026