Your boiler has detected a problem with the gas valve not closing correctly, so it has shut down the burner for safety.

Burner in fault position. Fuel valve closes too late. Check the gas train and both control cables.

What causes this fault?

This fault occurs when the boiler's internal computer detects that the gas valve is taking too long to shut off or is failing to close completely after a heating cycle. The most common reason is a mechanical failure within the valve assembly itself or a communication error in the wiring harness that prevents the signal to close from being received instantly. For safety reasons, the boiler identifies this as a risk and prevents the burner from firing again until the issue is resolved.