BoilerFaultCodesPro AL26 Fault Code
The Pro AL26 fault code means: your boiler has sensed that the water returning from your radiators is too hot, which usually means the heat isn't being transferred into your rooms properly.. It appears on 1 Pro model in our database.
Technical description: Return temperature too high
DIY fix possible
Check that at least 2 or 3 radiator valves (TRVs) are turned to the highest settingEnsure all radiator valves are fully open if you have recently closed themCheck that the bypass valve, if visible, hasn't been accidentally adjusted
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Pro models that show the AL26 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Pro AL26 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