BoilerFaultCodesGlow-worm 1 2 Fault Code
Also written as 12 — same fault.
The Glow-worm 1 2 fault code means: your boiler has stopped working because the water pressure inside the system has dropped too low to operate safely.. It appears on 1 Glow-worm model in our database.
Technical description: Loss of system pressure, i.e. less than 0.7bar. (Display will repeatedly flash '1' then '2')
What causes the Glow-worm 1 2 fault?
This fault occurs when the water pressure inside your heating system drops below a safe level, causing the boiler to shut down to prevent damage. The most common reasons are a slow leak from a pipe or radiator, or a loss of air in the expansion vessel which helps manage pressure changes. If you have recently bled your radiators, this can also cause the pressure to drop enough to trigger this code.
This issue is frequently seen at the start of autumn when homeowners turn their heating back on after the summer and find that small, slow leaks have lowered the pressure over time.
DIY fix possible
Locate the filling loop (a flexible silver hose) underneath or near the boiler.Slowly open the valves on the filling loop until you hear water flowing.Watch the pressure gauge and close the valves once it reaches 1.5 bar.
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Gas Safe Register →Glow-worm models that show the 1 2 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm 1 2 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 27 April 2026