378 High

Viessmann 378 Fault Code

The Viessmann 378 fault code means: your boiler is attempting to light but is failing to keep the flame burning steadily, causing it to shut down for safety.. 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 Flame loss during stabilisation or operating phase
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 378 fault code

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

Source

Viessmann 378 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