F.400 High

Viessmann F.400 Fault Code

Also written as F400 — same fault.

The Viessmann F.400 fault code means: your boiler has detected an issue with the flame detection system, meaning it cannot confirm the burner is operating safely and has shut down to prevent gas from building up.. It appears on 4 Viessmann 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: Burner in a fault state | Lead break, ionisation voltage, signal 1
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.400 fault code

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

Source

Viessmann F.400 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