BoilerFaultCodesVaillant d.1 Fault Code
Also written as D1 — same fault.
The Vaillant d.1 fault code means: your boiler is briefly keeping the internal pump running after the heating has switched off to cool down the internal components.. It appears on 9 Vaillant models in our database.
Technical description: Heating system pump over run
What causes the Vaillant d.1 fault?
This occurs because the boiler needs to dissipate residual heat from the internal heat exchanger once the heating or hot water demand has met its target. The system purposefully keeps the pump spinning for a few minutes to circulate water and prevent the internal components from overheating. It is a normal safety function of the boiler's operating cycle rather than a mechanical failure.
DIY fix possible
No action is required as this is a normal operational status code.Wait for a few minutes for the 'd.1' code to disappear automatically.Ensure your radiator valves are open to allow heat to dissipate.
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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.
- Vaillant Ecomax Condensing 600/800 series manual (PDF)
- Vaillant ecoMAX System 612 manual (PDF)
- Vaillant ecoTEC 65kW manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026