BoilerFaultCodesGlow-worm 0.4bar Fault Code
Also written as 04BAR — same fault.
The Glow-worm 0.4bar fault code means: your boiler has lost water pressure and needs to be topped up to work correctly.. It appears on 1 Glow-worm model in our database.
Technical description: Flashing pressure of 0.4bar or less, requires pressurising the system
DIY fix possible
Locate the filling loop (a silver flexi-hose) beneath the boilerOpen the valves at both ends of the filling loop to allow water into the systemWatch the pressure gauge and close both valves firmly once the needle reaches 1.5 bar
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Glow-worm models that show the 0.4bar fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm 0.4bar fault code information on this page is sourced and cross-checked against the official Glow-worm service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026