BoilerFaultCodesVaillant D.4 Fault Code
Also written as D4 — same fault.
The Vaillant D.4 fault code means: your boiler is displaying a status reading for the warm start sensor temperature rather than an error, which is part of the system's normal operational monitoring.. It appears on 5 Vaillant models in our database.
Technical description: Current temperature for warm stat sensor
What causes the Vaillant D.4 fault?
This code appears because the boiler is communicating its current internal temperature reading for the warm start function rather than an actual error. It physically occurs when the system is monitoring the thermal store to ensure instant hot water is available. You are seeing a live status update of the sensor's data, which means the internal computer is working exactly as intended.
DIY fix possible
Press the 'back' or 'menu' button to return to the main pressure or temperature displayMonitor the boiler for any actual fault codes starting with 'F'Ensure the warm start function (if desired) is enabled or disabled via the control knob
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Gas Safe Register →Vaillant models that show the D.4 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant D.4 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