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 3 Main models in our database.
Technical description: Primary water pressure is less than 0.5 bar
DIY fix possible
Locate the external filling loop, which is a flexible braided hose under the boiler connected to two valves.Open the valves slowly to allow water into the system until the pressure gauge on the front of the boiler reaches between 1.0 and 1.5 bar.Ensure both valves are fully closed and check the pressure gauge to confirm it is stable.
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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.
- Main Eco Compact manual (PDF)
- Main Eco Compact Combi manual (PDF)
- Main Eco Compact System manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026