ok so i installed a header, and highflow cat on my ep.
check engine light came on after about 20 miles of driving.
im pritty sure its one of the o2 sensors .
they were rusted pritty bad.
does the car still run the same with that light on or does it slow me down?

 

The car will likely not have any adverse effects due to faulty O2 sensors. My primary O2 sensor came disconnected when I installed my header, and I had to drive a few days with the wires disconnected and I only noticed poor gas mileage. What code is it throwing? Is it P0420, P0421, P0430, or P0431? I got and still periodically get P0420 after my header install and once I correctly re-wired the O2 sensor it corrected. You will need a code reader to read and reset the code once you repair the cause. Build an O2 sim and you will notice less problems.