We purchased an '08 Impala earlier this year from CarMax. It's a decent ride, I suppose, but nothing to write home about.
The stereo is the base model (AM/FM/CD/AUX) without controls for Sirius/XM or CD Changer.
Meh. It works.

After getting the first oil change since we purchased the car, the TPMS started acting funny. It was reading pressures from different tires. My guess is that it was a result of rotating the tires.
So CarMax took care of the TPMS issue by recalibrating the TPMS.
It works fine now.

However, since this work was done, the stereo stays on after pulling the key for A LOT longer than it used to. When we bought the car, it would stay on about the same amount of time as the courtesy lighting, 30sec or so. Now it stays on for at least 5min.

I've been through all of the informational/diagnostic menus that are available, and been through everything I can think of on the headunit itself. Obviously, I've come up with nothing.

Any ideas?
Is this something that I can change myself?
Or is this something that will have to be fixed by hooking it up to a computer to reprogram something?

Thanks in advance!