BoilerFaultCodesAtag 104 Fault Code
The Atag 104 fault code means: your boiler has detected that water is not circulating properly through the system, which could be due to trapped air, low pressure, or a blockage.. It appears on 2 Atag models in our database.
Technical description: Flow Check Failed
DIY fix possible
Check the pressure gauge and top up to 1.5 bar if it is lowBleed all radiators to remove trapped air from the systemEnsure all radiator valves and isolation valves under the boiler are fully open
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Atag models that show the 104 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Atag 104 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