Low
d.23 on Vaillant ecoMAX 635

Your boiler is currently identifying that the external control dial used to switch between heating and hot water modes is not functioning correctly or is in an unexpected position.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

What the manual says

Manufacturer manual

Summer/Winter function (control knob)

From this model's manual
Verified against the Vaillant Ecomax 613 installation & service manual, page 47.
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

Locate the temperature control knob on the front of your boiler panel.Rotate the dial fully to the left and then fully to the right to ensure it is not stuck or caught between settings.Reset the boiler using the dedicated reset button if the code persists.

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