F.104 High

Viessmann F.104 Fault Code

Also written as F104 — same fault.

The Viessmann F.104 fault code means: your boiler has received an 'alert' signal from an external part of your heating system, such as a smart thermostat or a safety accessory, which has told the boiler to stop running.. 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: External fault message input active
Typical repair cost £70-180 (including labour)

DIY fix possible

Check your thermostat or smart home app for any error messages or low battery warnings.Ensure any external pumps or safety switches attached to the system are powered on.Restart the boiler by pressing the reset button to see if the external signal clears.

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Viessmann models that show the F.104 fault code

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

Source

Viessmann F.104 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