how do i determine what's putting my 37X in limp mode
 

2013 G37X sedan with 26k miles on the car. Bought it used in 2016 with 6k miles (came off a lease like brand new). It's never had a sick day since I've owned it.
2-1/2" FI catback exhaust and HFCs installed and car driven for 200 miles afterward with no issues.

Bought a Stillen SC and did the install myself. First I installed the fuel pump supplied with the kit and drove it around for a while, with full throttle pulls to redline and no issues.
There were several issues during the install requiring rework of parts but got it installed. Not very confidence inspiring, and not at all the level of quality I expected from Stillen. I won't go into it in this thread but if anyone is interested PM me.
After completing the SC install and pushing the base tune map with UPREV, the car would not start. Seemed to be a fueling issue with several cylinders at a minimum. After exhausting all the troubleshooting I could come up with, I pulled the manifold and fuel rail, tested fuel pressure (50 PSI with a Matco analog gauge) then tested the individual DW injectors that came with the Stillen kit. The DW 650cc injectors all measured at .012 ohms. With the USCAR to Denso adapters supplied by Stillen connected to the same injectors, 4 of 6 injectors were electrically wide open. F*#%ked by $20 worth of bad parts. Retailer (Z1) overnighted some better adapters and after reassembly the motor fired up first try. Things were looking up. I reinstall the front end (quite a fight being as the Stillen charge pipe is the wrong shape and pushes against the rad stay despite all the material I Dremeled off the car), dropped it off the QuickJacks, and took it for a test drive. It produced 3.8 PSI of boost and pulled with authority until hitting a wall at 4k RPM as if it's in limp mode. After a few minute of lugging it and trying not to hit the soft 4k RPM rev limit, I hear knocking at 1500 to 2500 RPM and 10 to 20% throttle opening (best estimate). I put it in manual mode and keep it between 2500 and 3900 RPM but by 10 minutes into the drive the power is reduced to that of a John Deere garden tractor. All the while, no SES light, no codes, no pending codes. I logged 2 short drives and upon examining the logs from each session, the AFR seems okay but the knock strength goes from zero to over 1200 suddenly, minutes into the drive, and the ignition timing goes as advanced as 50 degree BTDC (which is insanely advanced in my experience).

Local shop says it's usually cam position or crank position sensors but I have no idea which. I never touched the connectors for any of those during the install. It seems quite a coincidence for any of the sensors to go now.

Question is, how is the ECU in limp mode if there are not even any pending codes?

How do I determine what's ailing it? Drive it more until it records codes?

Any insights are appreciated.

My 2nd waking thought this morning (after realizing I had not really inheritied 1000 acres of rolling hillside in Western Pennsylvania where I was going to build a cabin and grow weed) was that I could probably log the output of all of the suspect sensors and look for anomalies. Not sure why it took a day and a half to think of it. Worth a shot anyway.

There are no options to log any sensors except for VVEL.
Back to zero options.

What kind of code reader are you using? It's rare for limp mode to not have any codes being triggered.

I agree, it begs the question, if there are no sensory errors, how can it be in limp mode?

I used the OBD2 function built into the UPREV software.
The SES light was not on at the time. It revealed one code after one of the test drives (P1734, a transmission error) which I cleared. Since there was a code stored but the SES light was not on, I wonder if it was a pending code even though it was not labeled as such.
I used a FIXD wireless OBD2 scanner. No codes.
I went to a friend's shop to use his Snap-On scanner on Friday but he was closed for Good Friday so I headed over to my buddy's house and checked with his Cen-Tech scanner that he's been relying on for years. That was the only one that let me selectively view pending codes. No codes, pending or otherwise.

I'll try the Snap-On tomorrow.

Turns out there's nothing wrong with my car other than poor communication.
The tuner never told me he was limiting the engine to 4k. I was following Stillen directions which instructed me to do 3 WOT 3rd gear 7k pulls to verify the max boost.
I discovered it this morning when I logged data including max rev limit. There it was staring me in the face. Unbelievable.
If I had been told that this was the case, either initially or when I emailed the tuner FIVE days ago, I would have not wasted lots of my time and some of yours.

Runs pretty good now, though... got in some WOT pulls to 5k. Hit 6 PSI peak boost.
Apparently I'm going to be given 1k more revs periodically.
Maybe I'll be able to redline it in a few weeks.