When I put it in D to pull uphill out of the driveway it felt like I was in 4th gear. I shut the engine off and it was okay on restart. About a mile later it repeated and the fault code came on with a little alarm tone. It's back in the garage now, and when I was maneuvering to back in I had first gear available and no code showing. It has shifted smoothly up to this point.
My AE software will arrive Tues and I hope it can read the code, the dealer is over 30 mi away and I don't want to drive it again without knowing what's wrong.
Could low fluid cause this problem?

 

sounds precisely like an A-drum failure, reverse usually works fine, but you'll be in 'restricted performance' mode 9 out of 10 times. Be careful, the ring has either cracked apart or broken off if it truly is the common drum failure, caught early, no additional internals are damaged.

 

I have only had the car for a week, but it came with a six month drivetrain warranty, so I'm not upset.
If it has to go to the dealer should I have it trailered, or is it okay to drive in limp home mode?
I had planned on replacing the a drum myself next winter, maybe this is going to save me a lot of work.

 

The AE program had these stored codes:
P1798 CAN TCM/ Instrument Pack Circuit Malfunction
P0790 Normal/Performance Switch Circuit
P1797 CAN TCM to ECM Malfunction
P1799 CAN TCM/ ABS Control Module Malfunction
I think these all point to connection or module problems rather than A drum or other mechanical issues, but I don't have any experience in dealing with transmission codes.
I won't be able drive the car again until I install the new chain tensioners which are on the way, so I don't know if the codes will repeat.

 

well, definitely go looking at the harness and connector at the modules and transmission. IIRC, the a-drum failure doesn't spit out a code that points directly to it, it just generates a fault that is a result of the incorrect combination of drums to equal the proper gear. All seem to point to the TCM as the common module. It may be easier to diagnose if first you found a donor TCM that was compatible, and did a swap? Maybe members around here have one you could try? Would be great if a rebuild of the TCM is all you need...but you'll still need to work on the internal eventually.