BoilerFaultCodesAtag FILL Fault Code
The Atag FILL fault code means: your boiler has lost enough internal water pressure that it cannot operate at full capacity, so you need to top it up to restore normal heating and hot water.. It appears on 7 Atag models in our database.
Technical description: Water pressure is to low (<0,7 bar), flashing FILL will alternate with indication of water pressure, boiler power of 50% is possible. The installation needs to be topped up.
DIY fix possible
Locate the filling loop (usually two silver valves connected by a braided metal hose) underneath the boiler.Open the valves slowly to allow water into the system until the pressure gauge reads between 1.0 and 1.5 bar.Close both valves tightly to prevent over-pressurisation and check for any minor leaks.
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Gas Safe Register →Atag models that show the FILL fault code
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Source
Atag FILL 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