I got the dreaded 2885, and started troubleshooting.
The dealer had first crack at it - not under warranty - and replaced the air filter to turbo intake pipe, and the error returned on their test drive.
The dealer's recommended next step was to replace the turbo - at $3400 list, which they were willing to discount to $1900 (generous discount, but still... jaw meet floor). Presumably the turbo swap was to fix a potentially bad wastegate or actuator, but they weren't exactly clear. That being a little out of my budget, I did the turbo swap myself - upgrading to a JCW turbo (rebuilt, from JMT) at the time. Total cost ~$1k and about 2 weeks of time. The thermostat module was the most adventurous thing I'd done to date (more on that in a moment...), so doing the turbo was a stretch for my mechanic skills going in. That said, it went as smoothly as I could have expected. About 30 miles into the test drive, the error returned.
Researching the next best option, I was reading the 2885 troubleshooting guide over at the JMT site. Remember the thermostat housing swap I mentioned? I remember the vacuum lines being in the way for that, but they wouldn't detach easily so I left them in place and worked around them, and very likely damaged them in the process. I opted to swap those next. There's exactly nothing on the interwebz or in the Bentley book about replacing N14 turbo vacuum lines.
75 miles after that fix, and I'm starting to think that the problem is fixed! Since it seems to be boost / acceleration related, those 75 have been pretty spirited, punchy miles, and it's still running like a champ. So, problem fixed, boostier turbo in place, learned a lot about my car, got a few new tools, and still came out ahead of the discounted dealer quote - I think I'll call that a win.
