105 Low

Atag 105 Fault Code

The Atag 105 fault code means: your boiler is currently running an automatic air-bleeding cycle to remove bubbles from the system, which typically happens after a power cut or when first switched on.. It appears on 7 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: Venting program active when power turned on or interrupted
Typical repair cost £0 (including labour)

DIY fix possible

Wait approximately 7 to 10 minutes for the 'filling and venting' cycle to complete automatically.Check that your radiators are bled and the system pressure is between 1.0 and 1.5 bar.If the code persists for over 15 minutes, try restarting the boiler via the on/off button.

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Atag models that show the 105 fault code

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

Source

Atag 105 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