Okay. So I installed my new cat and cat back today. Couldn't get the o2 sensor out of the old cat, so in running without one since technically I can't drive anyways. Test drove it around my neighborhood, everything was perfect. Drove a across the street to work, .75 miles never exceeding 35 mph, everything is fine. Obviously my check engine light is one from having no oxygen sensor in, so it's heading towards limp mode but it isn't quite there yet. Drive to the bank for my boss, and instantly it's running rough. Determined that there's an exhaust leak between the flex pipe and the cat. Not a big deal. But the car is running rough and like ****. I'll have the o2 sensor in by tomorrow or tonight anyways. Not really worried about anything since I'm not driving too far anyways. My question is, is it running rough because its in limp mode, or something to do with the leak?