Medium
F39 on Viessmann Vitodens 025-W

Your boiler has lost connection with its indoor thermostat, meaning it cannot correctly measure the temperature inside your home to control the heating.

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

Lead break, room temperature sensor

From this model's manual
Verified against the Viessmann Vitodens 025 W installation & service manual, page 60.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

DIY fix possible

Check if your wireless thermostat has run out of batteries and replace them if necessaryEnsure the thermostat is within range of the boiler and has not been moved too far awayCheck for any loose wiring if you have a visible wired wall thermostat

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