So I got my first CEL yesterday As I was pulling out of the parking lot after work I realized the light was on, so promptly pulled back into the lot and called OnStar. I was given the code U2605, which means the "Battery Energy Control Module Lost Communication with Hybrid/EV Battery Interface Control Module 3." The first technical person I spoke with figured it was just some computer error and suggested that I try powering off the vehicle, waiting a minute, restarting, and then call OnStar back to see if the error code went into the history. I tried that, even going so far as to exit the vehicle while it was powered down for 2-3 minutes, but according to the OnStar folks I talked with the second time the DTC was still active. The funny thing was that when I called OnStar the second time, the level one guys suggested that I take it into the dealer within 5-7 days but the higher level tech guy said to take it in within 24 hours. This seemed a little extreme to me since the car appeared to be working fine otherwise, and the first tech guy I talked with seemed to suggest it would probably go away after 3 trip cycles.
Since the car was working I went ahead and drove home (30 all-EV miles) and arrived without incident, plugging in to charge right away. After a couple of hours I needed to move my car out of the garage even though I didn't have a full charge, so I unplugged, powered on, and moved the car only to find the CEL was still lit. In the morning I went to check out my car and lo and behold the CEL was out. Just as the first tech suggested, three cycles and it went out. We'll see if it comes back, but it appears to have been a non-issue in my case. I should be going in to the dealer in the next week or so for the battery enhancement, so if it happens again I can always alert my dealer at that time.
Since I wasn't able to find too much help on my error code (or the related ones U2603, U2604) on this site I figured I would go ahead and report my non-event here just in case this happens to anyone else.
2011 #130: 31,050 EV miles / 35,746 total miles (turned in 12/21/13)
2014: 10,863 EV miles / 11,130 total miles