F.27 on a Vaillant boiler means: flame signal detected while the burner is off. The boiler has seen a flame signal when there shouldn't be one — for example before ignition or after the gas valve has closed. It shuts down as a safety precaution.
This code applies to Vaillant ecoTEC. Codes can mean different things on different models, so check the display against your boiler's user manual.
When to call an engineer
This needs a Gas Safe engineer. It can point to a gas valve not closing properly or a fault in the flame sensing circuit. Don't keep resetting the boiler.
Never remove the boiler casing yourself — everything behind it must be handled by a Gas Safe registered engineer. If you smell gas, open windows, turn the gas off at the meter, and call the National Gas Emergency number on 0800 111 999.
Repair it, or replace it?
If your boiler is over 10 years old, compare a repair against a new combi before you pay for parts.
Applies to: Vaillant ecoTEC
Meaning checked against: Vaillant — boiler fault codes
Last reviewed November 2026. Always follow your boiler's own user manual.