OK, so it started off that I couldn't "Read entire" PCM without the Nav system cycling.
Removing the Onstar / Audio fuse resolved this and I was able to complete the read operation.
Since then, I have re-installed the fuse, and with the fuse in place, wrote a calibration. This worked fine.
I made some additional changes, and wrote a second calibration just fine.
Didn't like how it ran compared to the first change, so I started writing a 3rd tune to the PCM when it hung-up 70% through.
The Nav started cycling again, and after a few min with it going no where, I pulled the fuse again.
It didn't help this time, and now I lost communication with the PCM and it won't even try to turn over.
I pulled the battery, waited, and re-connected it. Tried a write entire (Non-Active Test Write) it gets to about 50% and stumbles, but eventually makes it through. However the checksum fails at the end, and the car still won't turn over or do anything.
Hooked up a battery charger to make sure that wasn't an issue, but no go.
There is a faint high pitch noise coming from under the hood that I've never noticed before, not sure if it's related.
What now? PCM Fried?