The boiler is detecting a flame signal when there should not be one, so it has shut down for safety.
Most common on: ecoTEC plus, ecoTEC pro
F.27 means the boiler has detected a flame — or a flame-like signal — at a time when the gas valve should be closed and no flame should be present. Because a flame that will not go out is a safety concern, the boiler shuts down.
In practice it is usually a false or 'simulated' flame signal caused by moisture, a faulty flame-sensing electrode, or a leaking or slow-closing gas valve. It needs a Gas Safe engineer to test the electrode and gas valve rather than a reset.
Press reset to rule out a one-off glitch, such as brief condensation on the electrode.
F.27 relates to flame detection, so if it returns after a reset, stop and book an engineer rather than clearing it repeatedly.
The flame-sensing electrode, wiring and gas valve need testing by a Gas Safe engineer — this is not a DIY fix.
Work on the gas side of a boiler must be carried out by a Gas Safe registered engineer. The checks above are limited to safe, non-invasive things a homeowner can look at (pressure, resets, the gas supply) — if in doubt, stop and call an engineer.
TradePlanr turns the callout into paid work — log the job, quote the parts and labour, raise the gas safety record and send the invoice from your phone, even with no signal. Free to start, works in your browser, no download needed.
The boiler is detecting a flame signal when there should not be one, so it has shut down for safety. F.27 means the boiler has detected a flame — or a flame-like signal — at a time when the gas valve should be closed and no flame should be present. Because a flame that will not go out is a safety concern, the boiler shuts down.
You can try a single reset and the safe checks below, but Vaillant F.27 usually needs a Gas Safe registered engineer. Never work on the gas side of a boiler yourself — it is illegal and dangerous unless you are Gas Safe registered.
A recurring F.27 means the underlying cause has not been fixed — most often moisture on the flame-sensing electrode or its wiring. Resetting repeatedly will not solve it; get the boiler diagnosed properly.