Permanent DTC- how they are cleared

[chessy56]

A P026D has been kicking my butt for a while now. Despite trying several things, thus far I've been unsuccessful and am resorting to a subscription to Mitchell1DIY for troubleshooting assistance. What I'm now seeing are the usual "pending" codes, some of which have set the CEL and left me with a freeze frame of data (which I'm logging). My Autel 319 scanner has now picked up a "Permanent" P026D. Unless I'm mistaken, it would appear that not only can I NOT clear this code, but until the problem is fixed I cannot even complete a drive cycle as this error seems to re-occur (often times when idling or coasting in traffic). Questions:
1. Am I correct that neither I nor the dealership can clear a Permanent error- and only the ECM itself can clear it?
2. Assuming I find the source of this DTC error, can I still attempt a couple of drive cycles successfully if the error does not re-appear (in any form- even if it doesn't set the CEL)?
3. Any tips on what to look for? Yeah, this has been covered a few times in the past and the P026D is a nuisance. I've changed my engine air filter, changed the fuel filter, did the fuel line flush procedure, tried loosening the fuel filler cap (and reversed the gasket on this cap which provided me a noticeably better seal at the filler neck), checked for CAC leaks (of which, the OEM cold-side CAC pipe is leaking- an oily film covers the CAC behind that connection- but no DTCs due to loss of boost). I've been watching/logging other data, including the commanded/actual fuel pressure (both from the lift pump and the HP pump- neither of which are showing more than 1% difference from required pressure). The 2 NOx sensors appear to be playing well as they both drop to "0" when in a decel mode (off ramp from the freeway) and the DEF system must be working as I typically see ~70% reduction in NOx ppm (or better) during normal operation. I have 2 injectors that appear to be slightly higher during the balance rate test compared to the others (ex: 1 being 0.7 or 0.4 when the rest are +/-0.2, with cylinder 4 & 6 being the 2 that stand out). I wanted to pull the connector from the fuel pressure sensor that resides behind the throttle body on the cold air pipe side to look for corrosion per a TSB I've found, but it looks like I'd have to pull the throttle body AND remove the alternator to get my stubby hands in there (it's "tight", with 2 harnesses directly in front of that connector).

So- any other tips would be of great help. I could ask the dealership if there's been any updates in the system s/w that might address this, but I'm not banking on anything. In the meantime, I'm going to have to keep clearing the CEL (so that my regens will happen) and start pouring through the Mitchell data to see if I can find anything else. Last resort is a trip to the dealership for a 1 hour charge to perform an "initial" diagnosis....

Any Help?