Your boiler has sensed a flame when there shouldn't be one, or the sensor that monitors the fire is sending a confusing signal to the control board.

Flame detection fault

What causes this fault?

This fault occurs when the boiler's control board detects an electrical signal suggesting a flame is present even when the gas should be turned off. The most common reason is a faulty sensing electrode that has became damp or coated in carbon, though it can also be caused by a leaking gas valve or a glitch in the main circuit board.