Your boiler's computer has detected a flame when it shouldn't be there, or is confused by an electrical signal from the burner area.

Fault: Electronics/flame - Implausible flame signal

What causes this fault?

This fault occurs when the boiler’s control board detects an electrical signal suggesting a flame is present even though the gas should be turned off. The most common reason is a hardware failure within the main circuit board or moisture and carbon build-up on the sensing electrodes causing a 'ghost' signal. It is essentially a safety conflict where the boiler shuts down because it cannot confirm the burner is actually extinguished.