Here's what came up for that code:

Definition:
H02S21 circuit condition (heated oxygen sensor bank 2 sensor 1)

Explanation:
The Powertrain control module uses the oxygen sensor to calculate the Air/Fuel ratio in the exhaust.

Probable Cause:
-O2 sensor defective
-Fuel system running very rich or lean - check other codes first
-Engine misfire condition - repair
-Fuel pressure very high or low - fuel pump or pressure regulator

This came on shortly before I deployed, and I did not mess with it. The car is heavily modified, and was most likely tuned (yes, I know it's bad that I don't know everything about it, but it was modded and then sold by a dealer). I had the car in for a cooling fan issue and they ran the diagnostic and all that. Would the dealer doing their test wipe a tune and thus cause a O2 code? I have not inspected the O2 sensors yet, but I will start there. Just wondering if anyone else has experience with this code and if so what fixed it for them. Also, I had put a new battery in when I got back and obviously this reset the code. I believe it came on after 9 days or so. I have not noticed any change in performance yet.