Your boiler thinks a flame is still present even though the gas should have been turned off, which is a safety concern that prevents it from running.

Flame detected after gas shut off.

What causes this fault?

This fault occurs when the boiler's internal sensor continues to detect a flame even after the system has commanded the gas supply to close. The most common reason is a gas valve that is physically stuck open or leaking, though it can also be caused by a sensing electrode or control board incorrectly reporting fire when none exists.