Your boiler has shut itself down for safety because it has detected a significant electrical or mechanical issue with its internal components.

Volatile lockout - Check Temperature sensor (8000-20,000Ω), Low voltage wiring harness / connections to sensor, Fan / fan harness / connections to fan, Code plug fitted.

What causes this fault?

This fault occurs when the boiler's internal computer, the Printed Circuit Board, suffers a critical failure or receives abnormal electrical signals it cannot process. The most common reasons are a malfunctioning circuit board, a faulty ignition system that is confusing the controls, or a sudden electrical surge that has damaged the sensitive internal components.