Boiler
F11
The F11 error code on a Vaillant boiler indicates a flame loss fault — the boiler has either failed to ignite or the burner flame has cut out unexpectedly. This is a moderate-to-serious fault that will lock out your boiler and leave you without heating or hot water until it’s resolved.
What does this error mean?
The F11 code means your Vaillant boiler attempted to fire up but could not establish or maintain a stable flame at the burner. The boiler’s control board detected no signal from the flame sensor (ionisation electrode) within the expected window and triggered a safety lockout. This is a protective shutdown — the boiler is working as intended by refusing to run without confirmed ignition.
Est. repair cost
£120–£350 including Gas Safe engineer labour and parts (electrodes, gas valve, or PCB depending on diagnosis)
🔍 Likely causes
🛠️ Step-by-step fix
Check your gas supply first: confirm other gas appliances (hob, gas fire) in your home are working. If they aren’t, contact your gas supplier — this is not a boiler fault.
If you have a prepayment gas meter, top it up and wait a few minutes for the supply to restore before proceeding.
In cold weather (below 0°C), check whether your condensate pipe — the plastic pipe that runs outside — is frozen. If it is, thaw it carefully with warm (not boiling) water and a cloth. Vaillant’s own guidance supports this as a homeowner task.
Once any external issues are ruled out, attempt a boiler reset: press and hold the reset button (the flame/reset symbol on most Vaillant ecoTEC models) for 3–5 seconds until the boiler attempts to restart. Allow two full ignition attempts before concluding the reset has failed.
If the F11 code returns immediately after reset, do not attempt further resets — repeated failed ignition cycles are not safe and will not resolve an underlying component fault. Stop here and call a Gas Safe registered engineer.
📞 When to call a professional
Call a Gas Safe registered engineer if the F11 code returns after one or two reset attempts, if your gas supply is confirmed working, or if you suspect a component fault such as the electrodes, gas valve, or PCB. Tell the engineer: ‘My Vaillant boiler is showing an F11 lockout fault — it’s losing flame or failing to ignite.’ Never attempt to open the boiler casing, adjust the gas valve, or interfere with burner components yourself — this is illegal for unqualified persons and potentially lethal.
⚖️ Repair or replace?
A Gas Safe engineer callout for an F11 fault typically costs £80–£180 depending on your region, with parts (electrodes, gas valve, PCB) adding £40–£350 on top. If your Vaillant boiler is under 10 years old and otherwise reliable, repair is almost always the right call. If it’s over 12–15 years old and this is one of several recurring faults, a new A-rated Vaillant ecoTEC or equivalent (installed cost £1,800–£3,000) will deliver better efficiency and reliability long-term.