High Lockout
d3 on Worcester Bosch ZWB 7-25

Your boiler has detected an interruption in a safety circuit, usually meaning a wire or link is disconnected, which prevents the heating from turning on.

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

Jumper 8-9 not detected or external limiter has been activated.

What causes this fault?

This fault occurs when the boiler's safety circuit is interrupted, acting like a tripped fuse to prevent the unit from getting too hot. It is usually caused by an external safety thermostat detecting high temperatures or a loose connection in the wiring link on the main control board.

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

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