F.78 High

Viessmann F.78 Fault Code

Also written as F78 — same fault.

The Viessmann F.78 fault code means: your boiler's main control panel is struggling to communicate with the internal system, which is preventing the boiler from functioning correctly.. 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: Normal operation Communication between heat management unit and programming unit faulty.
Typical repair cost £150-350 (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.78 fault code

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

Source

Viessmann F.78 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