BoilerFaultCodesAtag 78 Fault Code
The Atag 78 fault code means: your boiler has detected that the water pressure is either too low or too high for it to operate safely, or the sensor that monitors this pressure has lost its connection.. It appears on 7 Atag models in our database.
Technical description: Water pressure outside of range or not connected
DIY fix possible
Check the pressure gauge on the front of the boiler or via the digital display.If the pressure is below 1.0 bar, use the filling loop valves to carefully top up the system to 1.5 bar.If the pressure is above 3.0 bar, bleed a radiator to release excess water and pressure.Reset the boiler once the pressure is within the normal operating range (1.0 to 1.5 bar).
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Atag models that show the 78 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Atag 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