So I have a 07 g6 gt 3.5. I replaced the cats with walker cats due to the cats being bad. After I started the car and it was struggling to run and missing really bad. It threw a p300 code. It will start then the rpm goes up and down and it dies. My friend I hooked the battery like a idiot and now it won’t throw the code again but is doing the same thing. Can’t figure it out any ideas?