F.666 Medium

Viessmann F.666 Fault Code

Also written as F666 — same fault.

The Viessmann F.666 fault code means: your boiler is having difficulty communicating with a sensor responsible for managing water heating efficiency, which may prevent your hot water from heating up properly.. 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, Lead break, 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.666 fault code

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

Source

Viessmann F.666 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