F1 Low

Atag F1 Fault Code

The Atag F1 fault code means: your boiler is notifying you that a specific brand of system protector or anti-freeze chemical has been detected in the water to help prevent internal corrosion and freezing.. It appears on 2 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: Anti-freeze in combination with protector (Fernox F1 / Alphi 11)
Typical repair cost £0-80 (including labour)

DIY fix possible

Press the Reset button on the control panelMonitor the display to see if the code clearsIf the code persists, check your system pressure is between 1.0 and 1.5 bar

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Atag models that show the F1 fault code

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

Source

Atag F1 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