BoilerFaultCodesVaillant F4 Fault Code
The Vaillant F4 fault code means: your boiler has failed to keep the flame lit and has locked itself out for safety.. It appears on 2 Vaillant models in our database.
Technical description: Ignition fault (lockout) Went out when lit
What causes the Vaillant F4 fault?
This fault occurs when the boiler successfully lights but then immediately loses the flame signal, causing it to shut down for safety. The most common reasons are worn-out sensors that can no longer 'see' the flame or a gas valve that isn't providing a steady enough flow of fuel to keep the burner lit.
DIY fix possible
Check that your gas supply is turned on and other gas appliances are working.If you have a pre-payment meter, ensure you have sufficient credit.Press the Reset button (the flame symbol with a cross) for one second to see if the boiler restarts.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Vaillant models that show the F4 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant F4 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