BoilerFaultCodesVaillant F.21 Fault Code
Also written as F21 — same fault.
The Vaillant F.21 fault code means: your boiler has stopped working because it cannot light the flame or keep it burning properly.. It appears on 4 Vaillant models in our database.
Technical description: The pilot flame has failed.
What causes the Vaillant F.21 fault?
This fault occurs when the boiler's control system attempts to light the burner but fails to detect a stable flame after several attempts. The most common reasons are worn-out ignition components that can't create a spark or a faulty sensor that fails to tell the boiler the flame is actually lit.
DIY fix possible
Check if your gas credit has run out or if other gas appliances in your home are workingPress and hold the reset button (the flame symbol with a cross) for 5 secondsWait 15 minutes and try resetting a second time if it fails again
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.21 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant F.21 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