BoilerFaultCodesAtag C 78 Fault Code
Also written as C78 — same fault.
The Atag C 78 fault code means: your boiler is struggling to circulate water, either because there isn't enough water inside the system or because the internal pump has stopped working.. It appears on 2 Atag models in our database.
Technical description: Water pressure outside of range or not connected or if water pressure is OK: Pump failure
DIY fix possible
Check your boiler's pressure gauge and ensure it is between 1.0 and 1.5 bar.If the pressure is low, use the filling loop to top up the system to the correct range.Check that your heating circulating valves are open and not blocked.
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Atag models that show the C 78 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Atag C 78 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