BoilerFaultCodesPro AL79 Fault Code
The Pro AL79 fault code means: your boiler has reached an unsafe temperature because the water inside cannot move around the system quickly enough to cool down.. It appears on 1 Pro model in our database.
Technical description: Flow temperature too high, temperature differential too high
DIY fix possible
Check that all radiator valves are fully openEnsure the boiler pressure is between 1.0 and 1.5 barSwitch the boiler off and back on again to reset the system
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Pro models that show the AL79 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Pro AL79 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