D.004 Medium

Vaillant D.004 Fault Code

Also written as D004 — same fault.

The Vaillant D.004 fault code means: your boiler is struggling to accurately detect the temperature of the water it is trying to heat, which may result in your hot water being inconsistent or unavailable.. It appears on 7 Vaillant models in our database.

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description: Measured value of hot water sensor
Typical repair cost £120-220 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Vaillant models that show the D.004 fault code

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

Source

Vaillant D.004 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