Your boiler has detected that the gas valve is not closing as quickly as it should, preventing the unit from shutting down the burner safely.

Gas valve shut off delay. Gas valve leaking, electronics defective

What causes this fault?

This fault occurs when the boiler's internal computer detects that the gas flame is staying lit even after it has commanded the gas valve to close. The most common reason is a physical stickiness within the gas valve itself or a communication error from the main circuit board that prevents the fuel supply from being cut off instantly.