Medium
12 on Vaillant ecoTEC exclusive

Your boiler is not detecting the connection to your wall-mounted thermostat control, meaning it doesn't know when to turn the heating on or off.

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

24V DC room thermostat

From this model's manual
Verified against the Vaillant Ecotec Exclusive installation & service manual.
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 that your wall thermostat has fresh batteries installed.Ensure the thermostat display is active and not showing a connection error.Check the wiring connection at the boiler terminal block for any loose or disconnected wires.

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Related fault codes

F.11View →F.13View →

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