Here's the description from OnStar:
I contacted my dealer before Christmas and they couldn't get me in. I need to make a quick 700-mile trip this week. I don't mind emissions junk too much, but I don't want to get caught out of town with a worsening situation because I needed to head down the road.The Emissions System is not performing as expected. An issue has been detected in the Exhaust Emissions System which monitors and controls exhaust gases released into the air from the engine. If the vehicle is continually driven with this light on, the emission controls might not work as well, the vehicle fuel economy might not be as good, the engine might not run as smoothly and could lead to future repairs.