What to check, in order
- Don't keep resetting it
Vaillant says this is a case for an expert, and trying to solve it yourself can be dangerous.
- Call a Gas Safe registered engineer
Or Vaillant on 0344 736 0049. Have the boiler's model and the code ready.
When to call a Gas Safe engineer
Vaillant lists a gas solenoid valve that isn't sealing properly, or a faulty circuit board (PCB), as the causes.
A repair visit will usually check the gas supply, the ignition electrode and the flame sensor.
- Note the model and serial number. They're on a label on the data badge, usually under the boiler or behind the front cover.
- Write down the exact code, and what the appliance was doing when it appeared.
- Check whether it's still under warranty. If it is, the repair may cost you nothing.
Quick answers
What does the Vaillant F27 fault code mean?
The boiler sensed a flame when the burner should have been off.
Can I fix Vaillant F27 myself?
Engineer only (engineer visit). Start with: don't keep resetting it; call a Gas Safe registered engineer. Vaillant lists a gas solenoid valve that isn't sealing properly, or a faulty circuit board (PCB), as the causes.
Is Vaillant F27 worth repairing?
Deal with it now. A gas valve that may not be sealing is a safety matter. Have it checked rather than resetting it.
Checked against
Last reviewed October 2026. Manufacturers update their pages, and your manual is the final word. Spotted a mistake? Email contact@appliance-error-codes.com.