High Lockout
D.12 on Vaillant ecoTEC plus Regular

Your boiler is having trouble communicating with the pump that heats your hot water cylinder, meaning you might not have hot water.

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

Cylinder charging pump via accessory module

What causes this fault?

This fault occurs when the boiler's main control board loses its connection with the external pump responsible for heating your hot water tank. The most common reason is an electrical communication failure within the plug-in module or a physical fault with the pump itself, preventing the system from diverting heat to your water cylinder.

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.

Find a Gas Safe engineer
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Related fault codes

D.13View →

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