BoilerFaultCodesKeston FU Fault Code
The Keston FU fault code means: your boiler has detected that the water inside it is getting dangerously hot because it cannot circulate through your radiators effectively.. It appears on 2 Keston models in our database.
Technical description: DIFF GREATER THAN 50ºC. CHECK ISOLATION VALVES ARE OPEN, CHECK PUMP, CHECK RAD VALVES OPEN, CHECK SYSTEM BLOCKAGES
DIY fix possible
Check that the radiator valves on your radiators are open.Ensure the isolation valves underneath the boiler pipes are in the open position.Check if your central heating pump is running or has become seized.
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Keston models that show the FU fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Keston FU fault code information on this page is sourced and cross-checked against the official Keston service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026