Your boiler detects a flame when it should be turned off, which prevents the system from starting up safely.

Flame signal present with burner off

What causes this fault?

This fault occurs when the boiler's sensors detect a flame signal even though the burner should be currently switched off. The most common cause is a faulty ionisation electrode sending a false signal to the computer, or a gas valve that is failing to close fully and allowing a small amount of fuel to continue burning.