Medium
F.666 on Viessmann Vitodens 100-W WB1A

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.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

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.

What the manual says

Manufacturer manual

Preheating of second DHW cylinder and solar transfer pump not working, Lead break, temperature sensor, DHW preheating TS3

From this model's manual
Verified against the Viessmann Vitodens 100 W Wb1a installation & service manual, page 99.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Related fault codes

F.667View →F.668View →

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