It will come down to following a troubleshooting protocol. I would hope that this included swapping positions or exchanging units between aircraft. If you were unlucky you could have a pool of rogue FAC.
I am not an engineer but I do remember some obscure problems with Airbus 300-600 fuel computers that caused faults and were returned by the overhaul company with no fault found on bench testing. They would fail on fit when put back into the aircraft and I don't think that the exact cause of the failures was ever discovered before the aircraft were retired. Sometimes they would work fine on the ground but a few hours of cold soak soon started the failures again.
as to final responsibility it would be difficult to reject an aircaft for service if the log shows no faults but I would read the corrective actions in the log very carefully to see what fault finding had taken place.
The A320 family relies on engineers interrogating systems through the mcdu to troubleshoot systems and a lot of the old fashioned engineering practices and knowledge are slowly becoming a lost art,imho.
Last edited by tubby linton; 3rd Feb 2015 at 00:18.