BoilerFaultCodesVaillant D.173 Fault Code
Also written as D173 — same fault.
The Vaillant D.173 fault code means: your boiler has stopped working because the water pressure inside the system has dropped too low to operate safely.. It appears on 3 Vaillant models in our database.
Technical description: Minimumpressurelevel
DIY fix possible
Locate the external filling loop, which is a flexible braided hose usually found underneath the boiler.Open the two valves at either end of the hose until you hear water flowing.Watch the pressure gauge on the front of the boiler and close the valves once the needle reaches between 1.0 and 1.5 bar.
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Vaillant models that show the D.173 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant D.173 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 7 July 2026