Medium Lockout
D.27 on Vaillant ecoTEC

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.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

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 this 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.

Engineer required

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

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Related fault codes

F.27View →D.28View →F.70View →

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