D.40 Low

Vaillant D.40 Fault Code

Also written as D40 — same fault.

The Vaillant D.40 fault code means: your boiler is displaying the temperature it is currently aiming for rather than a fault, which is a normal status message and not a breakdown.. 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: Flow temperature target value

What causes the Vaillant D.40 fault?

This code appears when the boiler's internal computer calculates the specific temperature it needs to send to your radiators to meet your heating demands. It is physically triggered by the system's sensors and controls communicating the required heat level based on your thermostat settings. Because this is a standard status update rather than a failure, your boiler will continue to operate as normal.

DIY fix possible

Check if your thermostat or external controls are set higher than the current room temperatureWait for the boiler to complete its current heating cyclePress the 'i' button to exit the diagnostic information menu if you entered it accidentally

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Vaillant models that show the D.40 fault code

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

Source

Vaillant D.40 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