D.28 Medium

Vaillant D.28 Fault Code

Also written as D28 — same fault.

The Vaillant D.28 fault code means: your boiler is having trouble communicating with an external control or smart home accessory, which might prevent your heating or hot water from turning on correctly.. It appears on 19 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: Switching accessory relay 2

What causes the Vaillant D.28 fault?

This fault occurs when the boiler's main internal computer loses communication with an external control box or smart home wiring module. The most common reason is a loose connection in the wiring or a technical failure within the accessory module itself that tells the boiler when to turn on and off.

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.28 fault code

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

Source

Vaillant D.28 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