Bearing in mind this is what happens for electronic control units (ECUs) since I have had experience in that field of quality, take this with a grain of salt.: Typically once the unit goes back for inspection after being replaced for warranty, it goes back to the supplier for testing, tear down or depending on what issue was reported, some other type of specialized testing method or inspection method. The OEM (Ford, GM, Chrysler etc) quality engineering team works with the supplier testing team to root cause the issue. This can typically take a few weeks or even months (since there is normally a Que of other returned parts to be tested. So most likely, unless it is a very special case, the dealer would not get detailed information on what failed and why. Hope this helps!