F.667 Medium

Viessmann F.667 Fault Code

Also written as F667 — same fault.

The Viessmann F.667 fault code means: your boiler is having trouble communicating with the sensors responsible for heating your hot water storage tank, which means your secondary hot water supply or solar heating system may not be functioning correctly.. It appears on 2 Viessmann models in our database.

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description: Preheating of second DHW cylinder and solar transfer pump not working, Short circuit, temperature sensor, DHW preheating TS3
Typical repair cost £150-250 (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.667 fault code

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

Source

Viessmann F.667 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