02 sensors

so, i have a 99 mazda b3000 that i removed the cat backs on, after i did so the check engine light went on because of the post cat back 02 sensor and im wondering if this effects my gas mileage or a/f ratio

I would think this would affect both. Gas mileage is easy to keep track of. Does it seem like it's running ok? I would think after awhile the engine will run rough if you don't get that taken care of.

well it seems to run fine, and it already has an 02 sensor right after the manifolds that supposedly, controls a/f so i dont know exactly why there would be a post cat back 02 sensor