BoilerFaultCodesVaillant F.4 Fault Code
Also written as F4 — same fault.
The Vaillant F.4 fault code means: your boiler is failing to light the flame properly, which means it cannot heat your radiators or water.. It appears on 4 Vaillant models in our database.
Technical description: Ignition fault.
What causes the Vaillant F.4 fault?
This fault occurs when the boiler's control system attempts to start a flame but fails to detect one after several attempts. The most common reasons are a lack of gas reaching the burner, worn-out ignition sparks that can no longer bridge the gap, or a carbon-coated sensor that cannot 'see' the flame once it has lit.
This fault can be more frequent during the first cold snap of autumn when internal components that have sat idle all summer finally seize or fail under heavy use.
DIY fix possible
Check if your gas supply is turned on at the meterCheck if other gas appliances in your home are working correctlyPress the reset button (flame symbol with a cross) for one second
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Vaillant models that show the F.4 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant F.4 fault code information on this page is sourced and cross-checked against the official Vaillant service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026