D.23 Low

Vaillant D.23 Fault Code

Also written as D23 — same fault.

The Vaillant D.23 fault code means: your boiler is currently set to summer mode, which means it is only heating your tap water and has temporarily turned off the central heating.. It appears on 28 Vaillant models in our database.

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.

Technical description: Summer/Winter function

What causes the Vaillant D.23 fault?

This status occurs when the heating system has been manually or automatically switched to its summer setting to save energy. It physically prevents the boiler from sending heat to your radiators while still allowing it to provide hot water for your taps and showers. The boiler is not actually broken; it is simply following an instruction to prioritise hot water over room heating.

This code is most frequently seen during the spring and summer months when homeowners turn off their heating as the outdoor temperature rises.

DIY fix possible

Locate the radiator temperature dial or button on the boiler control panelTurn the dial clockwise or press the '+' button to increase the target heating temperatureCheck that the display no longer shows 'Summer' or the radiator icon with a cross through it

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Vaillant models that show the D.23 fault code

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

Source

Vaillant D.23 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 27 April 2026