While diagnosis is unlikely the number one issue is always a contact or connector. Followed by some chip death. Diagnosis was something I did years ago in a military setting where we used high powered microscopes and in a pinch the SEM (see google.) We also were known to take it to a micro-slicer machine to shave it down to the failure point micron by micron until we found it. It was all great fun and detective work but my point is that it is far cheaper to put in new parts than diagnose it.