Truck not starting and need help

[jak.sm]

Hello all,
I am doing my first engine swap, going from a 4.8 to a 5.3 ls, and I ran into a pickle. I completed the swap, made sure everything was working and everything was wired up correctly. I went to go turn the key over and nothing happened. I checked everything again and tried turning the key over again and still nothing. I tried turning the engine over with the starter relay and the engine turns over and it'll start for two seconds and die after. I have checked every fuse, relay, made sure the engine is receiving fuel, and about every other thing you can think of. The truck isn't throwing a VATS code or anything. I've been in this pickle for a while now. Can anyone help? I appreciate any advice in advance.

[5FDP]

So this is just an engine swap right, nothing else?

Same computer and same wiring harness correct?

And all you did was change the tune for the 5.3 swap correct?

[2xLS1]

Even if you didn't change anything in the PCM it would crank, start, and run with the 4.8 calibration. Even if it was the wrong engine Gen it would at least crank. I would bet you didn't hook up a ground somewhere.

[jak.sm]

So this is just an engine swap right, nothing else?

Same computer and same wiring harness correct?

And all you did was change the tune for the 5.3 swap correct?

Yes, just a engine swap, same wiring and computer. I had a tuner look at it but he said he couldn't do much cause the ignition key was turning the engine over or anything at all.

[jak.sm]

Even if you didn't change anything in the PCM it would crank, start, and run with the 4.8 calibration. Even if it was the wrong engine Gen it would at least crank. I would bet you didn't hook up a ground somewhere.

I made sure everything was wired up and grounded. I even had a mechanic look to see if I had missed a ground or wired anything wrong.

[2xLS1]

Did you have the transmission out? There is a neutral safety switch on the side of the transmission.

[jak.sm]

Did you have the transmission out? There is a neutral safety switch on the side of the transmission.

I did and checked to make sure it wasn?t damaged or anything before plugging it back in. Should I replace it?

[2xLS1]

No don't just randomly change parts. Troubleshoot the circuit.