Your boiler has detected a flame when there shouldn't be one, which has caused the system to shut down for safety.

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What causes this fault?

This fault occurs when the boiler's internal sensors detect a flame after the gas valve has supposedly closed or before the ignition sequence has even started. The most common reason is a sensor that has become covered in carbon deposits and is sending a false reading, or a gas valve that is failing to shut off the fuel supply completely.