D.27 Medium

Vaillant D.27 Fault Code

Also written as D27 — same fault.

The Vaillant D.27 fault code means: your boiler's internal control board is having trouble communicating with an external part or accessory, which may prevent the system from starting up 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 1

What causes the Vaillant D.27 fault?

This fault occurs when the boiler's main control board fails to detect or communicate with an internal switching relay or an external accessory. The most common reason is a loose connection in the wiring or a physical failure of the relay component that manages how the boiler talks to external parts. In some cases, it can indicate that the main circuit board itself can no longer send signals properly.

Typical repair cost £180-350 (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.27 fault code

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

Source

Vaillant D.27 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