Boiler
F2
The F2 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 during operation. This is a moderately 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?
When your Vaillant boiler displays F2, it means the appliance attempted to light the burner but could not detect a stable flame via the flame sensor (ionisation electrode). The boiler’s control board runs a timed ignition sequence and, if no flame signal is confirmed within that window, it shuts down as a safety precaution. In plain terms: your boiler tried to fire up, couldn’t confirm the flame was burning safely, and locked itself out to prevent a gas build-up.
Est. repair cost
£80–£250 for a Gas Safe engineer repair (part + labour); a reset costs nothing if the fault is a one-off
🔍 Likely causes
🛠️ Step-by-step fix
Check your gas supply first: confirm other gas appliances (hob, gas fire) are working. If not, contact your gas supplier — this is not a boiler fault.
Check your prepayment meter (if applicable): top up if the credit has run out, then proceed to reset.
Perform a boiler reset: locate the reset button on your Vaillant boiler (usually a flame symbol with a line through it), press and hold for 3 seconds, and wait up to 5 minutes for the boiler to restart. This clears a one-off ignition failure.
Check for a frozen condensate pipe (common in winter): the condensate pipe runs outside and can freeze solid. You can safely thaw it by pouring warm (not boiling) water along the external pipe. Reset the boiler once thawed.
If the fault returns after 2–3 resets, stop here — do not attempt to open the boiler casing or touch gas components. This is now a job for a Gas Safe registered engineer.
While waiting for an engineer, ensure the boiler’s gas isolation valve (the inline valve on the gas pipe feeding the boiler) is in the open position (handle parallel to the pipe). If in doubt, leave it alone.
📞 When to call a professional
Call a Gas Safe registered engineer if the F2 code returns after two reset attempts, if you smell gas at any point (in which case call the National Gas Emergency line on 0800 111 999 immediately and leave the property), or if no DIY step resolves the fault. Tell the engineer: ‘My Vaillant boiler is showing fault code F2, it’s locking out on ignition, and a reset doesn’t hold.’ This helps them bring the right parts — likely an ignition electrode, flame sensor, or gas valve.
⚖️ Repair or replace?
A Gas Safe engineer callout and repair for an F2 fault typically costs £80–£250 depending on the part required — an ignition electrode or flame sensor at the cheaper end, a gas valve replacement at the higher end. 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 isn’t the first fault, get a replacement quote — a new A-rated boiler installed typically costs £1,800–£3,000 and will be far more efficient.