BoilerFaultCodesAtag C40 Fault Code
The Atag C40 fault code means: your boiler has detected that the water pressure inside the system is too low for it to operate safely.. It appears on 1 Atag model in our database.
Technical description: Water pressure fault
DIY fix possible
Locate the filling loop, which is a silver braided hose with one or two valves connecting your cold water mains to your heating pipes.Slowly open the valve(s) to allow water into the system while watching the pressure gauge on the front of the boiler.Stop filling once the needle on the gauge reaches the green zone, typically between 1.0 and 1.5 bar, then tightly close the valves.
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Gas Safe Register →Atag models that show the C40 fault code
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Source
Atag C40 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