Your boiler has detected that the gas valve is not closing as quickly as it should, preventing the unit from shutting down safely and causing it to lock out.

Gas valve shutoff delay defective; Gas fitting leaking, electronics defective

What causes this fault?

This fault occurs when the boiler's internal computer detects that the gas valve is failing to shut off properly after the flame should have extinguished. The most common reason is a mechanical stickiness within the gas valve itself or a failure in the main control board's electronics that incorrectly tells the valve to stay open.