12 Medium

Vaillant 12 Fault Code

The Vaillant 12 fault code means: 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.. It appears on 2 Vaillant 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: 24V DC room thermostat
Typical repair cost £90-180 (including labour)

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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Vaillant models that show the 12 fault code

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

Source

Vaillant 12 fault code information on this page is sourced and cross-checked against the official Vaillant service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026