BoilerFaultCodesKeston F4 Fault Code
The Keston F4 fault code means: your boiler has detected that a temperature sensor is not communicating correctly, meaning the system cannot tell how hot the water is and has stopped working for safety.. It appears on 8 Keston models in our database.
Technical description: Control / No flow thermistor
What causes the Keston F4 fault?
This fault occurs when the boiler's internal computer loses contact with the sensor that monitors the temperature of the water leaving the unit. The most common reason is that the sensor has either degraded due to constant heat cycles or the electrical connection between the sensor and the main control board has become faulty.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Keston models that show the F4 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Keston F4 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