iFixIt’s Kyle Wiens provides a superb rundown of the Error 53 issue, detailing the story so far as well as technical information. We’d expect nothing less from the Internet’s best repair site.
Apple’s been hiding behind the excuse that Error 53 is caused by “other invalid components” used when repairs are made by non-Apple personnel, with particular reference to the Touch ID sensor. iFixIt calls them on this, pointing out:
Error 53 isn’t necessarily a problem of third-party parts. It can happen with new OEM parts out of a different iPhone. It’s a matter of synchronization—not third-party parts.
In short, it’s a mess – and the kind of mess that Apple seems very good at walking into, even during the Jobsian era (remember Antennagate?)