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 air-removal cycle to clear trapped air from the system, which is a normal process after maintenance or a power restart.. 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: De-aeration program actif

DIY fix possible

Wait for the cycle to complete, which typically takes between 10 to 20 minutes.Ensure your heating and hot water are still turned off at your thermostat while the process finishes.If the boiler does not resume normal operation after 30 minutes, switch the power off and back on at the fused spur.

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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 27 April 2026