F.27 High

Vaillant F.27 Fault Code

Also written as F27 — same fault.

The Vaillant F.27 fault code means: your boiler has detected a flame when there shouldn't be one, which means the safety system has locked the unit down to prevent an unsafe situation.. It appears on 53 Vaillant models in our database.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description: Warm start sensor interruption (ecomax 800) / False light (flame signal present but gas valve closed)

What causes the Vaillant F.27 fault?

This fault occurs when the boiler's sensors detect a flame signal even though the gas valve should be closed and the burner turned off. The most common reason is a faulty ionisation probe sending a false electrical signal to the control board, or a gas valve that is slightly leaking or failing to close fully. In some cases, moisture on the internal electronics or a failing circuit board can also trick the system into thinking a flame is present.

This fault can sometimes be more frequent during periods of high humidity or heavy rain, as moisture ingress can affect the sensitive electrical signals between the probe and the control board.

Typical repair cost £120-£400 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Vaillant models that show the F.27 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Vaillant F.27 fault code information on this page is sourced and cross-checked against the official Vaillant service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026