I hate that feeling. I've been there with my own car as VW has spent between 10k and 12k on it during the CPO warranty, if I had to shell out that kind of money I would be not so happy. Now that the issues have been sorted out I really want to keep this one as it is in decent condition and I like it.I still have the Arteon R line. No news since dropping it off Friday 1/29. I don't think they have any real clue where the canbus fault is coming from.
Let's open up this conversation... what happens if they can't diagnose this? The car has Bilstein B14 coilovers, APR stage 1 tune, and a bunch of other very normal bolt on mods. The dealer has been working with me since day 1, and they have been great. everything has been covered so far. Listen, I WANT THIS CAR. I WANT THIS CAR FIXED. IF THEY TELL ME I NEED TO PAY FOR IT BECAUSE 'X reason and it's because x part' thats ok. The problem is they can't seem to find it. They can't see to diagnose this.
Theoretically, what's next? Do you see VW stepping up here since this is clearly not my fault, per say, but they COULD have an easy out. Thoughts?
As far as finding your issue, I'll make this as short as I can but the one of the weirdest CEL's I saw on a Cayenne was an S that somebody bought new and drove till about 30k miles when a recurring CEL(different codes) for the evap system started showing up. We put so many parts in that truck, everything remotely related to the evap system was replaced. Problem was, the CEL would always clear and test drive fine and leave for a few weeks to months at a time. We started to suspect something was else wrong pretty early on but the parts replaced always fixed the corresponding code so it went it's merry way. The last visit before we figured out what was going on the codes started to get really strange and didn't follow normal diagnostic trees. We got in touch with our regional tech specialist who was going to stop by while he was in the area to look at it. This guy was a genius, he had wiring diagrams memorized down to the wire color and was almost clairvoyant in his problem solving ability.
He spent a couple days with this Cayenne S checking this and that and running wires all over it to duplicate/check harnesses etc. He finally traced the issue to a cubby hole in the rear passenger hatch area. Apparently the owner had bought a trailer hitch receiver when it was new and tossed it in this cubby hole. Problem was, this cubby hole and it's lining sat on top of a body seam. Guess what ran over the body seam? A ground wire harness for the evap system. The weight of the hitch receiver moving/sitting on the wires was just enough to rub through the insulation over 30k miles and create an intermittent internal short. So after a simple wiring repair all the codes cleared and everything was back to normal. He kept the truck for a few more years and gave it to his son, never had another CEL for the evap system lol.
TLDR: Every problem, no matter how complex, has a solution, it could be very simple but the trick is finding it. There is nothing a good tech hates more is a comeback that is not his/her fault. It is just a matter of time but they will figure it out. There are very few unfixable issues, cars that I have seen go Lemon Law were often fixed in the right hands. That being said, what are your options in VA? Did you buy it used? Lemon Law's in most states don't apply to used cars. If it comes down to you getting a different car, could you take the parts off of it? The dealership would most likely work with you in that regard.
If this was a powertrain issue, your stage 1 tune would be a problem. I'm sure the car has been scanned multiple times and the tune has shown up on the VW side. If it were an issue, I'm sure you would know by now. The warranty admin at your dealership wants to make sure claims are paid and anything to jeopardize that would be brought up immediately.
I hope you hear some good news soon.