2007 Acura TSX, 2.4 Liter L4, with ~203k miles, and dual temp A/C controller.
The A/C has stopped cooling, so I pulled out my gauge set to see what I could discover.
It has not had any R134a added in a few years, but it appears to still be fully charged.
With the car not started, both LS and HS pressures read about 85 psi (80-F ambient).
When I start the car and turn on A/C, the compressor runs but the readings don't change.
It seems like it must be either a failed compressor, or a failed thermal expansion valve.
So the question is -> What test(s) can I perform to pinpoint which one is the problem..?
I am totally willing to tackle the replacement of either, but they're both pretty big jobs.
So I would really like to zero in on which one is causing no HS pressure to build...
Thanks!

 

Very nicely worded & good thinking testing the pressures not running & running & checking if the compressor is coming on.
Sorry to admit but my knowledge on the subject seems comparable to yours, as in you've done everything I know to do. BUT there's a guy on YouTube "ScannerDanner" who has great videos on car repairs & he's all about diagnosing the correct part.. His slogan is "don't be a parts changer", ya know, one of those ppl who needlessly replaces parts til they eventually figure out which was the culprit. I know he's got several on AC diagnostics, so if you're still unsure you could check him out. I'm not affiliated, just a big fan. I've used his videos to fix mine and my families cars any time something has came up and I've successfully repaired everything that's needed fixing just by watching his videos.
Good luck!