I hollowed out my cat the other day and now sometimes when I floor it, it sounds and feels like it is misfiring. The SES light came on and my brother ran the codes at his shop and its on because of the cat. Someone on another board told me to put an o2 sim from thunder racing in and it would stop. What is an o2 sim? Does anyone have any other theories of why my car is like this?

 

Hollowing out a cat will not cause a miss. That is likely a seperate problem. Depending on the year of the car, hollowing the cat will cause a check engine light. The code will be for low cat efficency. The computer will look at the pre and post cat voltages. It expects a difference in the voltages if the cat is doing it's job. Since your cat is hollow, it sets a code. The 02 sim will fool the computer into thinking the cat is still there by taking the place of the post cat sensor. You can also get rid of the check engine light through programming with LS1 edit etc. and turning the code into one that doesn't turn on the check engine light.