BoilerFaultCodesAtag c1 18 Fault Code
Also written as C118 — same fault.
The Atag c1 18 fault code means: your boiler has detected that the water pressure inside the heating system has dropped too low, so it is running at a reduced level to protect itself.. It appears on 4 Atag models in our database.
Technical description: Water pressure is too low (<1.0 bar). Power is reduced by 20%.
DIY fix possible
Locate the filling loop (usually a silver flexible hose) connected to your boiler pipes.Open the valves on the filling loop slowly to let water into the system.Watch the pressure gauge on the front of the boiler until the needle reaches the green zone, typically between 1.0 and 1.5 bar.Close both valves tightly and remove the filling loop if it is not a permanent fixture.
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Gas Safe Register →Atag models that show the c1 18 fault code
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Source
Atag c1 18 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