D.125 Medium

Vaillant D.125 Fault Code

Also written as D125 — same fault.

The Vaillant D.125 fault code means: your boiler is struggling to accurately measure the temperature of the hot water being sent to your taps, which may cause your hot water to fluctuate or stop working entirely.. It appears on 3 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: Domestichotwatercylinderoutlet temperature
Typical repair cost £120-250 (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.125 fault code

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

Source

Vaillant D.125 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