I have a 98 buick lesabre I am at whits end with I put a different motor in the motor before blew head gaskets and tied up same make model donor it took 6 months to swap due to cold weather in Ohio fired it up ran about 3 seconds and just stopped like you shut the key off security lights isn't on oil pressure is good tried to reset vats anyway because everything is pointing in that direction and still same issue used the pcm ecu from the car and tried ecm from donor still same issue anyone have idea please let me know

Issue solved guys and to many others with same issue it turned out during the process of motor swap the was a broken wire in the body plug 🤯it the blue wire in the upper right hand corner of the body plug fixed issue with a pin from old motors and a butt connector after that I in plugged the wire and it continues to start every time without it hmmm I just reconnected it to be safe and happy hunting for this pescky broken wires

Great information. Glad you found the wire & all is well now. Good Job !