I could not find anything on any other forum so I'm posting this FYI.
I replaced the ECM in my 2006 GXP because it was shutting off intermittently. I ordered an ECM online, had it flashed to my VIN, and installed it without any issue. My car started right up without any PK3 learning or anything at all. However, there was a message on my Driver Information Center, "Theft Deterrent System Not Programmed." Although I could clear it, it reappeared every time I started the car. The car ran fine but the remote start would not work and I continually had to clear the annoying message from the DIC display.
The dealer could not explain it so I researched the issue online with no avail. My solution was to patiently await the P0315 error code, Crankshaft Position System Variation Not Learned or CASE relearn procedure not started. It took about 2 months, 2000 miles for the error code to appear. Once done for $45, the DIC Theft Deterrent System message went away, everything is fine, and the car runs as good as new. There was one Chevy Forum post from about 2008 with a similar issue and that guy came to the same conclusion.
I hope this helps somebody. Peace.
