It might be a problem with the loops of sensors for the fire detection systems etc
For example: ECAM message "FIRE LOOP A FAULT" (ATA 49: APU)
Both engines and the APU each have two identical loops, A & B and a computer- FDU (Fire Detection Unit). A fire warning is given when both loops reach the proper overheat condition. If one loop fails the other loop is able to generate the warning by itself. A fire warning is given if both loops fail within 5 seconds of each other.
...
The leak detection system uses a single loop for the pylons and APU to detect hot air temps associated with duct leaks. Dual loops are used for the wings. If both of the dual loops detect a leak a warning is given, unless there is a fault on one, then only one loop is required to give a warning.
From Eric Parks training notes.
