BoilerFaultCodesVaillant D.1 Fault Code
Also written as D1 — same fault.
The Vaillant D.1 fault code means: your boiler is not broken; it is simply continuing to run the internal pump for a few minutes to safely cool down the heat exchanger after providing heating or hot water.. It appears on 14 Vaillant models in our database.
Technical description: Water pump overrun – time for heat mode
What causes the Vaillant D.1 fault?
This status occurs when the boiler has finished heating your radiators and needs to dissipate the high temperatures remaining inside the unit. The system keeps the internal pump running to move residual heat away from the heat exchanger and into the pipework to prevent overheating or stress on the components. This is a normal safety sequence and the boiler will automatically return to its ready state once the temperature drops.
This code is more visible during winter when the central heating is used frequently, as the pump needs to run more often to cool the system down after long periods of operation.
DIY fix possible
Wait for 5 to 10 minutesCheck if the display changes once the cooling cycle finishesEnsure your room thermostat is turned down if you want the pump to stop sooner
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Gas Safe Register →Vaillant models that show the D.1 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant D.1 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