BoilerFaultCodesKeston F6 Fault Code
The Keston F6 fault code means: your boiler has lost communication with the sensor that measures the temperature outside, which helps it run more efficiently.. It appears on 8 Keston models in our database.
Technical description: Outside sensor failure
What causes the Keston F6 fault?
This fault occurs when the boiler can no longer detect a signal from the small sensor located on the exterior of your home. The most common reason is that the sensor itself has developed a fault due to exposure to the elements, or the thin electrical wiring connecting it to the boiler has become damaged or loose. Because the boiler doesn't know how cold it is outside, it can't automatically adjust its settings to save you money on your heating bills.
This fault often comes to light during the first cold snap of autumn when the boiler attempts to communicate with the sensor to adjust for dropping temperatures.
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 F6 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Keston F6 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