Your boiler thinks a flame is present when it shouldn't be, causing the system to shut down for safety purposes.

Automatic Lockout. A flame is detected before an ignition attempt or unstable flame during run period.

What causes this fault?

This fault occurs when the boiler's control system detects an electrical signal from the burner even though it hasn't commanded the gas to ignite yet. The most common reasons are a contaminated flame sensor sending a rogue signal, a gas valve that is leaking slightly, or a fault within the main circuit board that is misinterpreting electrical noise as a flame.