Hi guys, I cross-posted this as a response to another thread, but I thought I would share some information that I hope will help, and create a new post. It has taken 2 weeks with my scan tool viewing the live data while I am driving around and I believe I have the solution. Once all the parts are in, I will know for sure, but everything makes sense. I too was getting erratic idling and intermittent CEL with codes P0172 & 5. It would go on and off with no rhyme nor reason. Luckily with our crazy midwest weather I was able to check things out at -10 temps and 70 degrees in these two weeks. A lot of consistency. Here is the deal.

Short Term Fuel Trims (STFT) were running 0 to 3.9 in all banks, which is essentially in spec. At times they were all zero'd out, which is perfect.
LTFT were running -30, but fluctuated from -8 to -35. When the value is over 30, it will set the codes off if it happens more than two drive cycles. The code would go on and off. but was memory resident.

In order to diagnose, I cleared the codes, and disconnected the negative battery cable for 30+ minutes while I cleaned the MAF sensor. After several drive cycles and watching the live data I was able to learn what this truck was doing. Mind you the MPG on some consistent long term trips over 100 miles would be 14 MPG one day and 21 the next. Today was the "ah ha" moment. Went to get the winning Powerball tickets and when I got back in the truck, I started it up and the CEL and the Stabilitrak light came on. While I was hooking up the scan tool the Stabilitrak light went out, CEL stayed. Checked codes and was the usual. I cleared the codes and sat and watched the STFT and LTFT. STFT was running -5 to -8 and the LTFT was -32. All banks were consistent. After driving around, I noticed that the shift timing was off, some harder up and downshifts. After a stop and some shopping, came out and it ran perfect to the next stop. I have learned that the STFT and LTFT can affect the TCU in the tranny and upset the shifting, torque patterns and can be responsible for the Stabilitrak perceived fault . During this event, I was able to grab some data and for the first time I caught the STFT at -18. So, STFT tells us what the truck is doing right now (and it was running rich), where the LTFT learns the vehicle over time. All work together to use the fuel efficiently (if you can say that in these trucks). The other thing to note was my Fuel Rail Pressure (FRP) and Command Pressure (CMD) were not aligned and not at all smooth. The numbers varied from trip to trip, with CMD at 485 and FRP bouncing from 405 to 630. This never smoothed out and they never aligned. If you think about it CMD is supposed to drive the FRP. I performed an injector leak down test, pressure was steady, so no leaks. I then did a drive test at varied hwy speeds. Fuel Trims both long and short stabilized at hwy cruising. One last thing I did, was check things at a cold start. LTFT and STFT were perfect, until things warmed up and then chaos ensued. So after some research and a little help from an AI subscription , I have come to the following conclusion. The High Pressure Fuel Pump (HPFP) is failing. The countless threads and bulletins that I read show me that this is classic failing pattern. Luckily my CAT temps are good and there is no fuel being dumped into the cylinders, as there is not a rich fuel smell. So I seemed to have caught this early.

My plan is to replace the HPFP and while I have things apart I am replacing the injectors. I will also be cleaning everything as I go and replacing PCV, and purge valve solenoid on top of this. All the parts cost me about $500 for OEM pump and injectors, aftermarket PCV and solenoid. I also purchased the fuel crush lines (stoopid one time use idea) the intake gaskets and the fuel pump install tool. Going to be doing the job when I het back from sunny Florida for Christmas, and if I happen to hit the PowerBall tonight, you will never hear from me again!

Just for reference, my truck is a 2016 Silverado LT Extended Cab, 4wd with 158,000 on it. I have meticulously maintained it, and recently had tranny rebuilt (~3000 miles ago). I am confident I have the correct diagnosis and I will report back once I replace and get a few drive cycles in. If anyone is interested, I have about 50 screen shots from the scan tool that I can share so you can compare to yours if you are having this problem. It all makes sense and with an intermittent problem, it was driving me nuts. Having a good scan tool with live data is essential with these older trucks.