the shop report can give some clues. as said before items that returned from repair with no fault found maybe havent been fixed. the J10 harness on the cfm56-5 is a good example. however it must be said that it might be difficult, for some parts, to replicate faults in parts in the shop and think that they are serviceable. i recall when we got the same faulty j10 harness 3 different occasions.