c 1 05 Low

Atag c 1 05 Fault Code

Also written as C105 — same fault.

The Atag c 1 05 fault code means: your boiler is currently running an automated process to remove trapped air from the system, which is a normal self-maintenance cycle and not a breakdown.. It appears on 1 Atag model 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: 7 minute de-aeration cycle begins (appears the moment the pressure exceedes 1.3 bar)

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 c 1 05 fault code

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

Source

Atag c 1 05 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