High Lockout
D.27 on Vaillant ecoTEC plus Regular

Your boiler's internal control board is having trouble communicating with an add-on electrical component that manages your external pumps or heating zones.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

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

Switching accessory relay 1

What causes this fault?

This fault occurs when the boiler's brain loses its electrical connection with the internal relay module that controls your external pumps and valves. It is usually caused by a loose wiring connection between these parts or a component failure on one of the circuit boards. Because the boiler can no longer safely coordinate with the rest of your heating system, it stops running to prevent damage.

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Related fault codes

F.27View →D.28View →

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