BoilerFaultCodesKeston L1 Fault Code
The Keston L1 fault code means: your boiler has shut down because the water inside it has become too hot, likely due to a circulation problem.. It appears on 8 Keston models in our database.
Technical description: Flow temperature overheat
What causes the Keston L1 fault?
This fault occurs when the water leaving your boiler exceeds a safe temperature limit, causing the system to shut down for safety. It is typically caused by a blockage in the pipework, a failing pump that cannot move the water quickly enough, or a build-up of debris known as sludge.
This issue often surfaces during the first cold spell of autumn, when the heating system is switched back on after being idle and internal components like the pump may have seized.
DIY fix possible
Check that your radiator valves are fully open to allow water to flow.Ensure your system pressure is between 1.0 and 1.5 bar on the pressure gauge.Press the 'Reset' button on the boiler control panel once the unit has cooled down.
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Gas Safe Register →Keston models that show the L1 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Keston L1 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 27 April 2026