Which scenario causes an amber half box to appear around the FOB on the upper ECAM display?

An amber half box appears around the Fuel On Board (FOB) indication on the upper ECAM display when there is an issue with the fuel transfer system, specifically due to a transfer valve failure. This alert serves as a caution that the fuel management system is not functioning as designed, potentially impacting fuel distribution between the tanks.

In the case of a transfer valve failure, the system identifies that one or more valves are not properly transferring fuel as needed, leading to an imbalance or inadequate fuel supply to either the center or wing tanks. The amber indication is a prompt for the flight crew to investigate and address the issue, ensuring that safety and fuel efficiency are maintained during the flight.

Other scenarios mentioned, such as pump failures or unreliable fuel quantity readings, would trigger different alerts or indications on the ECAM display but would not specifically result in the amber half box delineating the FOB. For example, while a loss of both wing tank pumps would affect fuel delivery, it typically raises a separate warning regarding the specific pumps rather than directly affecting the FOB display.