If it fails to autodetect on scan it should also do so on copy right?
Because if it is scan only, it would be some sort of corruption, and fix one would be to set the default to A4, do a scan, reboot, set the default as Auto detect, do a scan and reboot, then test to see if resolved.
But if it happens on copy and scan (And this is vastly more likely), then it is probably a hardware fault, and to diagnose that, you are going to need a service manual and tools and enough of an electrical background to be able to test the components within for the failure. And that is way beyond my ability to assist with.