swapped my 2002 Alero 4 cylinder for 2003 Alero 6 cylinder & it won't stay running

My boyfriend recently switched a 2003 Alero 6 cylinder engine with my 2002 Alero 4 cylinder engine. He installed the entire engine including the ECM harness. Everything seemed good until we tried to start it. It actually turns over & starts, but, dies immediately. I did make sure there was gas in the tank. HELP! We put a lot of work into the car & don't want to give up on it when it seems as though we're ALMOST THERE!

So, to be clear: My 2002 Alero (formerly a 4 cylinder) now has a 2003 6 cylinder Alero engine. Everything seems properly connected. It starts, but won't stay running. I would appreciate any feedback on what might get this thing running:confused:. Thanks in advance!

I'm going to bet it has something to do with the factory security/passkey. Need to figure out how to reset or bypass it.