you hit the header and damaged one of the o2 sensors. Run the code and it will tell you which sensor. It should be replaced. I'm pretty sure your gas mileage will suffer and the CEL will not go away till it is fixed.
Just take it into a good auto parts store like NAPA or Auto Zone, they will be happy to bring out their scanner and run the code for you at no charge.
That will at least give you a free starting point.
The front O2 sensor is real close to the block at the top front of the oil pan. Maybe it broke externally when the header hit? Not sure how hard you hit or if there would be that much movement but, should be able to see it.
I hit my header on a parking block and it broke my rear 02. I'm assuming it happened because it broke the support bracket as well. The front looks more likely to break, but looking under there a few days after I got the car back it's the rear sensor that looks new
The o2 sensor is between the cat and the tranny (?) if I recall correctly. The rearward thrust on the header sort of "pinches" (for lack of a better term) the o2 sensor and breaks it. Also, be careful, striking the header hard enough can cause broken side motor mounts.