D.035 High

Vaillant D.035 Fault Code

Also written as D035 — same fault.

The Vaillant D.035 fault code means: your boiler is struggling to switch between providing heating for your radiators and heating your hot water, which may result in a loss of one or both services.. It appears on 7 Vaillant models in our database.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description: Position of the diverter valve
Typical repair cost £180-300 (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.035 fault code

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

Source

Vaillant D.035 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