High Lockout
d3 on Worcester Bosch Greenstar 30HE

Your boiler has lost its safety connection to the external controls or the internal emergency stop circuit has been interrupted.

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

Link missing or external limiter tripped.

What causes this fault?

This fault occurs when the electrical loop that tells the boiler it is safe to run has been broken. The most common reason is a loose or damaged connection in the wiring to your external thermostat, or a fault within the thermostat unit itself which has stopped sending a signal to the boiler.

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
Search the official register →

Related fault codes

C1View →C6View →

Related guides you might find useful

Boiler keeps cutting out

Lockout fault with a reset-needed pattern.

Read →

Yellow or orange flame — CO safety

Safety-critical — turn off, read this.

Read →

When to replace your boiler

7 signs repair isn't worth it anymore.

Read →