High Lockout
d3 on Worcester Bosch Greenstar 12i System

Your boiler has lost its connection to an essential safety internal plug, meaning it currently cannot communicate correctly with its control system.

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

external cut-off switch activated

What causes this fault?

This fault occurs when the boiler's control board loses its physical connection to a specific internal safety plug that identifies how the unit should operate. The most common reason is that the plug has vibrated loose over time, sustained moisture damage, or the internal wiring harness has developed a fault. Without this connection, the boiler shuts down immediately as a safety precaution because it no longer recognizes its own configuration.

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

C1View →C6View →9AView →

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