BoilerFaultCodesPro AL40 Fault Code
The Pro AL40 fault code means: your boiler has stopped working because the water pressure in the central heating system has dropped too low.. It appears on 1 Pro model in our database.
Technical description: Insufficient system water pressure
DIY fix possible
Locate the filling loop (usually a silver flexible hose) underneath or near the boiler.Slowly open the one or two taps on the loop until you hear water entering the system.Watch the pressure gauge on the boiler until it reaches between 1.0 and 1.5 bar, then close the taps tightly.
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Pro models that show the AL40 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Pro AL40 fault code information on this page is sourced and cross-checked against the official Pro service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026