If the ECU doesn't see the crank sensor it WILL NOT engage the starter relay.

 

Its cranks just fine. It gets fuel as well.

I think I've narrowed it down to a bad ECM... which really sucks. I checked the IPDM, it's fine. I checked the BCM, apparently, it's fine as well. I'm not getting communication from the ECM which is very concerning.

Essentially, the crank sensor outputs the signal to the ECM, but the ECM doesn't send that signal to the coil pack signal wires to fire... So, I'm thinking that's my culprit. Or at least that's my best educated guess on this matter. lol

 

The ECU combines the signals from the crank and cam sensors to deliver spark/fuel. If any of them don't function it won't run.

 

Correct. I did replace all the sensors when I first got the vehicle, as I thought that was the issue. No dice.

However, when the previous owner swapped in this engine, I don't believe he disconnected the battery till halfway through the process. So, that may be reason to believe that something may have shorted the coil/injector control chip on the ECM... Maybe they accidentally turned the key on or something to that extent?

The joys of picking up someone else's project... haha Thank you for all of the input thus far, it's definitely been food for thought.

To have the ECU checked and repaired, it's going to be around $150-ish, so it would give me some peace of mind to know I have a good working ECM regardless if it's the culprit or not. I know they're rather fragile in these cars as it is. Let alone when some person just rips through it, then gives up on it. lol

 

As cleric670 and others have said, I would recommend using only Genuine Nissan OEM sensors or Hitachi for the Cam and Crankshaft position sensors. Cleric670 provided this advise to me when I was using aftermarket sensors and Hitachi sensors fixed the problem immediately. Before that I had installed aftermarket sensors from Amazon and Autozone and they did not work.