Your boiler has detected a sequence error or a flame signal where there shouldn't be one, causing the burner to shut down completely for safety.

Burner control switches to lockout

What causes this fault?

This fault occurs when the boiler detects a persistent flame signal even after the burner should have switched off, or if the internal electrical checks fail during start-up. The most common reason is a build-up of carbon or moisture on the ionisation probe, which tricks the system into thinking a fire is still burning. In some cases, a fault within the main control board or damaged ignition wiring prevents the boiler from safely managing the combustion process.