BoilerFaultCodesVaillant F1 Fault Code
The Vaillant F1 fault code means: your boiler has tried to start but could not create or detect a flame, causing it to shut down for safety.. It appears on 2 Vaillant models in our database.
Technical description: Ignition fault (lockout) Failed to light
What causes the Vaillant F1 fault?
This fault occurs when the boiler attempts to ignite but cannot confirm a stable flame is present. It is usually caused by a lack of gas reaching the burner, worn-out spark electrodes, or a buildup of debris on the internal components that prevents the ignition process from completing safely.
This fault can be more frequent during the first cold snap of autumn when old electrodes or gas valves struggle to operate after being idle over the summer.
DIY fix possible
Check if your gas supply is active by testing another gas appliance like a hob.Ensure your gas meter has credit if you use a pre-payment meter.Press the reset button (indicated by a flame with a cross through it) for three seconds.
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Search the official register for a qualified engineer in your area.
Gas Safe Register →Vaillant models that show the F1 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant F1 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