F.427 High

Viessmann F.427 Fault Code

Also written as F427 — same fault.

The Viessmann F.427 fault code means: your boiler has stopped working because it has detected an issue with an external control signal that tells it to turn on or off.. It appears on 1 Viessmann model 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: Burner in a fault state | Lead break, external unlock signal
Typical repair cost £150-300 (including labour)

Engineer required

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

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Viessmann models that show the F.427 fault code

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

Source

Viessmann F.427 fault code information on this page is sourced and cross-checked against the official Viessmann service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026