BoilerFaultCodesMain 109 Fault Code
The Main 109 fault code means: your boiler has detected that the water pressure inside the system is far too high for it to operate safely.. It appears on 2 Main models in our database.
Technical description: Fault code
DIY fix possible
Locate the filling loop under the boiler, which is usually a small silver braided hose with one or two valves.Ensure both ends of the filling loop are firmly closed, as a leaking valve is the most common cause of rising pressure.If the pressure remains high, vent (bleed) one of your radiators slightly to release excess water until the boiler pressure gauge drops to the green zone (typically 1.0 to 1.5 bar).
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Main models that show the 109 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Main 109 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 7 July 2026