BoilerFaultCodesKeston F5 Fault Code
The Keston F5 fault code means: your boiler has stopped working because it cannot properly measure the temperature of the water coming back from your radiators.. It appears on 8 Keston models in our database.
Technical description: Return thermistor
What causes the Keston F5 fault?
This fault occurs when the sensor responsible for monitoring the temperature of the water returning to the boiler begins sending incorrect data or fails completely. The most common reason is that the internal components of the sensor have degraded over time, though it can also be caused by loose electrical connections or a build-up of debris on the sensor's surface.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Keston models that show the F5 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Keston F5 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