Dodge neon Change engine won't start

mjengebretson spewed out this bit, and i'll scatter a few bits myself

One mistake I made when changing an engine in a 94 intrepid, I used the wiring harness that came with the engine I bought because I figured it would be the same. I just plugged the new engine harness into the large plug onside the shock tower, and it wouldn't start. I had to remove and install the wiring harness from my old engine, and it fired right up.

If your engine was running before the swap then it's either that or you've not got every thing plugged back in. Else start swapping out things from your old engine over to the new one (crank sensor and so on). You don't have to reset the computer.