Getting into Limp Home Mode is pretty hard, but it points to chip/memcal as Robert said. I've yet to see a chip or Memcal go bad without human intervention and even then it's hard.

VSS won't cause it, LHM is total failure backup. So back step what you did? Any pins in harness bad or bent, wiring, short. The Memcal didn't just go bad, something happened and it's probably fixable.