What does the Vaillant F27 error code mean?
F27 indicates the flame-sensing electrode is reporting a flame even though the gas valve should be closed. It is almost always a faulty flame-sensing electrode or moisture on the electrode/lead, and it's unsafe to ignore — call an engineer.
What causes the Vaillant F27 fault?
The F27 error on a Vaillant boiler is most commonly triggered by:
- A faulty flame-sensing electrode or damaged lead
- Moisture or corrosion on the electrode connector
- A PCB fault falsely reading the flame signal
- A gas valve that isn't closing fully
How to fix the Vaillant F27 boiler fault
Follow these steps. If the code returns after you have completed them, stop and call a Gas Safe engineer.
- 1.Switch the boiler off at the controls and do not attempt to run it.
- 2.Book a Gas Safe engineer — this is a gas-safety related fault and should not be DIY-repaired.
Frequently asked questions about the Vaillant F27
What does F27 mean on a Vaillant boiler?
F27 is a false flame signal: the boiler's flame sensor is detecting a flame when there shouldn't be one. It's typically a faulty electrode or lead, and occasionally a PCB or gas-valve fault.
Is F27 dangerous?
It can indicate a gas-valve or flame-detection problem that affects safe operation. Switch the boiler off and arrange a Gas Safe engineer — don't try to reset it repeatedly.