DGHook

I bought my z about a year ago and got it inspected but its about that time again... next Monday. Its a 2004 with nothing but bolt-ons. My list of mods include:

-Catch can on the PCV valve side
-Single cat-back exhaust
-R2C boxed off intake
-Z1 motorsports vacuum lines

So not much.

I've head the SEL on ever since I got the car, and have been trying to figure it out ever since.

The original code was a P0300 code for a random cylinder misfire, and I could
also feel the misfire but only at idle, which is what it's still doing.

So, I went about fixing it..(After everything I tried I reset the ECU)

The first thing I did was change my spark plugs and my coil packs, which did not solve the issue at all, but needed to be done regardless. When I changed them, in the #6 spark plug hole there was a brown liquid that looked like fuel, but didn't smell like anything. I figured it was just from the coil pack fluid that was in it because I've heard it has some in it.

Then, I changed my fuel injectors. When I changed my fuel injectors, as you all should know, you have to take off the whole plenum, so to avoid the possibility of the code being a vacuum leak, I replaced the intake manifold gasket and the plenum gasket. Which seemed to help but changed the code to the p1273 code and I can still feel the misfire.

The new code p1273 says the probable causes are:
-Failed AF sensor (looked up the code and found it was for the upstream oxygen sensors, not sure if bank 1 or bank 2)
-Vacuum leak in engine(I tested it for vacuum leaks, and there are none)
-Fuel injector or pressure regulator fault(haven't done anything with that information because I had changed my injectors, not sure about the regulator fault though)

So now my options are to either replace the oxygen sensors, or figure out whats up with the regulator fault, and I wanted some other z enthusiasts opinion before moving forward.
The other problem I have with the car is that the fuel gauge doesn't work, so I'm going to replace the fuel pump assembly and that should fix that issue, but wasn't sure if that could be effecting anything else and causing the code.
I change the fluids (oil, coolant, etc.) when they're supposed to be changed, and have no overheating or any type of issues like that.
I do all the work myself and I know for sure that everything has been installed right, so there is no mechanic error that could be causing the problem.
Thank you guys so much for reading and the suggestions you make, I really need the experienced among you to help me figure this out!