BoilerFaultCodesKeston F7 Fault Code
The Keston F7 fault code means: your boiler has detected that the electricity supply coming into your home is at a lower voltage than it needs to operate safely.. It appears on 8 Keston models in our database.
Technical description: Low mains voltage
What causes the Keston F7 fault?
This fault occurs when the boiler's internal sensors detect that the incoming electrical voltage from your mains supply has dropped below the minimum level required for the components to function. Most commonly, this is caused by a temporary fluctuation in the local power grid, though it can also indicate a failing printed circuit board (PCB) that can no longer process power correctly.
This fault is slightly more common during internal cold snaps or heatwaves when the national grid experiences high demand, potentially leading to 'brownouts' or voltage drops.
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 F7 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Keston F7 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