Your boiler detects a flame when it should be off, which is a safety error that prevents the system from starting up.

Flame On Before Gas Valve On

What causes this fault?

This fault occurs when the boiler's sensors detect a flame inside the burner assembly even though the gas valve should be closed. The most common reasons are a gas valve that is leaking slightly and failing to shut off fully, or a faulty sensor lead that is sending a false signal back to the main control board.