Q: 4l80e transmission in limp mode

Hello and thanks for your question. When changing out the transmission and ECM from another vehicle to your vehicle with the same year and model can effect many things. Check all plug to ensure that everything on the transmission is plugged in. Check the computer system to ensure all of the cables are tight and locked together. On most newer vehicles, the ignition switch is programmed with the computer system and is coded with the VIN number. In this case, you could do two things, either take the other ignition switch from the other vehicle and swap it out (only if the key has a chip), or have your computer flashed to the VIN on your vehicle. Also, try hooking up the original ECM to your vehicle, that is if the harnesses are the same and your old ECM is functional. As long as the same sensors are on the transmission as the other one, the ECM will not care what kind of transmission you have installed.