I have came on here because I have tried everything in my knowledge and everything YouTube has to offer and found no remedy. I have a 2006 Cadillac CTS that I have engine swap mini times. It originally A car that had been hit on the side and whenever I tried to start it it would crank and never start. Found out that the crank and camshaft sensors were thrown so I looked at those and one had snapped off and I took the motor apart and found that the camshaft bolt that holds it together had backed out and dug into the metal causing the engine to lock up which is probably why The original owner got into a crash. I cleaned everything up, changed the sensors, and redid the timing all to find out that the sixth cylinder had a misfire from bad rings or some thing else since it had blow by and a bunch of oil buildup on the piston head. Later on that engine blue and I swapped it with another one and it also wouldn’t start. I went back to the junkyard and got another car engine that I put in here and it started but had low oil pressure. Instead of messing around with junkyard motors I decided to buy a new motor off of eBay that had been remand. This is a reputable company and they have done their part as I inspected the engine before I put it in. I swapped it like all the others and now I have the engine in and I have been trying to figure out why The computer that was already on the car from the original engine would not start and also I’ve been getting a code saying that the computer has lost connection for the ECM. I tried buying a new ECM as I have had this problem before and tried to reprogram it but I cannot reprogram it and it still keeps giving me the same code about lost connection. I have checked the ground wires and they both have 12 V checking from my battery to the ground wire, I checked this on the actual ECU bolt that is a T 25, I have checked it on the boat that goes to the side of the engine, and I have also checked the ground that goes to the back of the engine. They all have the 12 V from the battery to the ground. I have replaced the ECU relay, I have also checked the fuses for the ECU none are blown and the relay I just purchased is good. My starter wires hooked up as I have also tried jumping the ECU fuse and heard the engine crank over and the engine did start for five seconds before shutting off because once again it had no connection to the ECU. The only thing this car does is give me the same code for lost connection with ECU and whenever I try to turn it on I hear the fuel pump prime and whenever I try to crank it i near The fuel pump turning on again but no cranking or starting. Finally I checked the wires that go from the Engine wiring harness to the BCM and from the fuse box to the BCM, put dielectric grease in both of them and still nothing.