D.15 Low

Vaillant D.15 Fault Code

Also written as D15 — same fault.

The Vaillant D.15 fault code means: your boiler is displaying an information code because the pump that moves water around your pipes is spinning at a different speed than expected.. It appears on 18 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: Pump speed actual value

What causes the Vaillant D.15 fault?

This code occurs when the boiler's main control board detects that the internal pump is spinning faster or slower than the speed it has been commanded to reach. It is usually caused by air bubbles trapped in the system or a build-up of debris like sludge that is physically slowing the pump's rotation. In some cases, it can also happen if the electrical signal between the pump and the main control board becomes inconsistent.

This issue is more common in autumn when heating systems are turned back on after summer, as settled debris or air pockets can migrate into the pump.

Typical repair cost £180-350 (including labour)

DIY fix possible

Check that all radiator valves are fully openEnsure there is enough water pressure in the system (check the gauge)Restart the boiler to see if the advisory code clears

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Vaillant models that show the D.15 fault code

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

Source

Vaillant D.15 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