Your boiler has detected a flame when there shouldn't be one, which means the internal sensors or ignition parts are sending incorrect signals to the control board.

Internal boiler wiring integrity or electrode deterioration.

What causes this fault?

This fault occurs when the boiler's control system detects an electrical current that mimics a flame, even though the gas hasn't been turned on yet. The most common reason is a buildup of carbon or moisture on the ignition electrode, which causes it to send a false 'fire' signal to the main computer. In some cases, it can also be caused by a fault within the internal wiring or the control board itself misreading the sensor data.