Under ambient factors of high temperature and high humidity, your vehicle may be susceptible to under hood wiring harness cracking on certain engine wiring harnesses. (fordcontour.org)
2014 chevrolet silverado z71 engine wiring diagram photos for help. (upcomingchevrolet.us)
It would help if we had the engine codes and the two ecu types (EDC15 / 16, Med7 / 9 ect) and the fault codes that are present but here goes: It sounds like the engine was swapped complete with the correct engine wiring loom for that engine and the correct ecu for that engine has then been unlocked (had the immobiliser removed). (mechanics.stackexchange.com)