BoilerFaultCodesMain 118 Fault Code
The Main 118 fault code means: your boiler has stopped working because the water pressure inside the heating system has dropped too low to operate safely.. It appears on 1 Main model in our database.
Technical description: Primary water pressure is less than 0.5 bar
DIY fix possible
Locate the external filling loop, which is usually a silver flexible hose connected to the pipes underneath your boiler.Open the small valve or tap attached to the filling loop slowly until you hear water flowing.Watch the pressure gauge on the front of the boiler and close the valve as soon as the needle reaches the green zone, typically between 1 and 1.5 bar.
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Gas Safe Register →Main models that show the 118 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Main 118 fault code information on this page is sourced and cross-checked against the official Main service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026