Permanent DTC- how they are cleared

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?

To paraphrase the book: :P026D sets when the fuel mass observer correction quantity exceeds emissions limits, as monitored by the NOx-1 sensor.

I saw a post on Reddit that said this trouble code was a result of the NOx-1 sensor sensing excessive fuel. Replacing the sensor fixed some of these issues, but not others, or the problem soon returned. Also, some argued that injector leakage was a likely cause. It's human nature to go to the expensive options first, but we should all eliminate the simple/less costly first.

Also...

Quote:

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Service Bulletin Bulletin No.: 20-NA-197 Date: September, 2020
P026D, P1089, P163A, P163B, P2293, P228A, P228B, P228C, and/or P228D Brand: Model: Model Year: VIN: Engine: Transmission: from/to Chevrolet Silverado 2017-2018 Silverado 2500HD/ L5P 3500HD 2019 2020 Silverado 4500HD, 5500HD and 6500HD 2019 2020 L5D GMC Sierra 2017-2018
https://static.nhtsa.gov/odi/tsbs/20...81720-9999.pdf

P026C / P026D (Malfunction of all injector (#1 - #6) Injection quantity and timing) A subject vehicle may exhibit an illuminated check engine lamp with DTC (Diagnostic Trouble Code) P026C and/or P026D. The engine ECU software may falsely detect an issue and record these DTC's in the engine ECU.
https://static.nhtsa.gov/odi/tsbs/20...17700-9999.pdf

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Depending on the individual code, trouble codes will generally reset by themselves once a prescribed number of ignition cycles have occurred without the problem presenting. I had a MAF code appear in Lil Red at the first start after replacing the head gaskets in 2021. Seems I didn't get the MAF connector fully seated during re-assembly. That SES light/code went away after a couple dozen ignition cycles. My reader couldn't reset the code for some reason in my conversion truck.