Your boiler has detected an electrical communication problem within its internal wiring which is preventing it from running safely.

check the wiring harness

What causes this fault?

This fault occurs when the boiler's internal components lose the ability to speak to one another through the electrical wiring. It is usually caused by a loose connection, a damaged wire within the main harness, or a failing computer board that can no longer process signals. Because the boiler cannot confirm that all parts are working safely, it shuts down as a precaution.