C1 05 Low

Atag C1 05 Fault Code

Also written as C105 — same fault.

The Atag C1 05 fault code means: your boiler is currently running an automatic cycle to clear trapped air from the system, which typically occurs after it has been powered on or refilled.. 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: Venting program active

DIY fix possible

Wait approximately 7 to 10 minutes for the automatic cycle to finishEnsure all radiator valves are open to help move air through the systemCheck the system pressure gauge and top up to 1.5 bar if it has dropped

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Atag models that show the C1 05 fault code

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

Source

Atag C1 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