Your boiler thinks it sees a flame even though it isn't currently firing, causing it to shut down as a safety precaution.

False flame lockout

What causes this fault?

This fault occurs when the boiler's internal sensor detects a flame signal at a time when the burner should be off. The most common reason is a faulty sensing electrode that is sending an incorrect electrical signal to the main control board, or moisture buildup on the electrical components causing a short circuit.