A1502 not connecting to working Apple SSD, boots to external drive
I have a A1502 I bought 2nd hand. It has been working find for about a year. Then it booted up one day and showed the flashing folder with ? that indicates it cannot connect to the ssd. I start up in recovery mode, and ssd is not seen in disk utility, or in startup disk. I can boot from an external drive (USB) or target disk (thunderbolt), so it seems the logic board is working fine. I have not upgraded to High Sierra. I get a rental drive Apple SSD from OWC to instal so that I can upgrade to High Sierra to upgrade firmware. I instal the rental Apple SSD, and laptop does not find that SSD either. I put the rental Apple SSD in OWC enclosure, and it can connect to it via USB.
Does this mean there is a connection issue from the SSD to the logic board? If so, it there a way to address just the connection without replacing the whole logic board? A new logic board (plus the new OWC drive I have already bought) would be more than I paid for the laptop second hand.
Any help or insight would be appreciated.