BoilerFaultCodesJohnson Starley F40 Fault Code
The Johnson Starley F40 fault code means: your boiler has detected that the water pressure in the central heating system is too low or too high for it to operate safely.. It appears on 2 Johnson Starley models in our database.
Technical description: Central heating water pressure fault
DIY fix possible
Check the pressure gauge on the front of the boiler or under the casing; it should be between 1.0 and 1.5 bar.If it is below 1.0, use the external filling loop valves to top up the system until the needle reaches the green zone.If the pressure is too high, bleed a radiator to release water and lower the pressure.
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Johnson Starley models that show the F40 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Johnson Starley F40 fault code information on this page is sourced and cross-checked against the official Johnson Starley service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026