actually the reason the check engine code comes on is because the o2 sensor is noticing that the air is moving through the exhaust at the same speed. the cat is normally there to slow down the movement. since the cat is gone, it moves through ratherquickly and throws the code. i was told the dealership can fix it, but you might as well take it and get a custom tune. for only 200-400 dollars, you can fix the check engine light, and get all of your parts workign together with freeing up some horse power. i need to make an appointment to get mine done. my light is on.