The engine light appeared Code P0400?

I tested the wire connecting from the EGR solenoid to the ECM and it is showing that it is "out of limits". Just to make sure that there is actually no signal coming from the ECM, I tested to see if there would be voltage from the wire connecting from the ECM to the EGR solenoid, and there was very little voltage despite the car being on idle. I checked the images you provided me, and it mentioned that the wire connecting to the ECM is supposed to have 11-12 volts when idle. I checked this by connecting one of the probes to the wire connecting to the ECM and connecting the other probe to ground. It showed less than 30 mV all the time.

TLDR; the wire connecting to the ECM showed it was "out of limits"

Since I know that the wire has a break, how can I go about locating this? Do I need to rent or buy specific tools for this?

I manually traced the wire, and there was no break in the wire according to the tool. I even made sure the pin to the ECU was good using the wire tracer, and it was. I don't understand why the multimeter it's showing 0F/open when testing the wire. Does this mean the ECU is faulty or is there something else I'm missing?

I checked all the EGR passages, and they’re all good. There is very little carbon buildup. This was the first thing I checked. I’m very certain it is an electrical issue since turning the car on didn’t change the air passageways on the solenoid when it should’ve.

I checked a different pin this time, and it showed that the wire is actually closed. So now I know the issue doesn't lie within the wire, but rather with something else.

I plan on replacing the ECM, but is there some other sensor besides the EGR Temp Sensor that I should check before replacing the ECM?

Also, if there is nothing else, I should check, how do you replace the ECM on this car? Do I have to buy a new/used ECM and get it programmed by the Nissan dealership, or can I use a different ECM with the same model number?

Is it possible to reprogram the immobilizer of the new ECM with a tool of some sort?

Or is it possible to reprogram it like in the video below:
https://youtu.be/tRbwMsUw8A8

I don't really want to go to a dealership and give them $200 to reprogram the ECM.

I think that procedure only works on a Toyota/Lexus.

Is there any other sensor I should check before I change the ECU?

Also, is there a tutorial on how to program an ECU and install it into your car? I plan on getting an ECU with the same serial number, but I don't know how to reprogram it and install it. I don't want the car to not start/run after I've installed the ECU.

I've been told that if I get an ECU with the same serial code, I don't have to flash it or anything like that, and I would just have to make the immobilizer work with the new ECU.

Just in case I get an ECU with the same serial number, how do I properly install it in the car? What are the steps I'm supposed to do when working with any scan tool to make the new ECU work with the car?

Can a leaky intake manifold cause the P1400 code to pop up?