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 signal error between its control system and gas supply, preventing it from lighting safely.. It appears on 46 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: No demand to gas valve | Cause: Electronic board defective, Gas valve defective

What causes the Vaillant F.27 fault?

This fault occurs when the boiler's internal computer detects a flame signal at a time when there shouldn't be one, or if the sensors are providing conflicting information. The most common reason is a communication failure between the electronic control board and the gas valve, causing the system to shut down as a safety precaution. In some cases, moisture or a failing component can trick the boiler into thinking gas is burning when the valve is actually closed.

Typical repair cost £250-£550 (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 7 July 2026