O2 sensor
 

I am having my 4-1 header installed in a couple of hours and i wanted to make sure of where the 2nd O2 sensor needs to be drilled. Please answer ASAP. Thank you

I had mine installed after High Flow CAt and b4 Exhaust. Hope that helps.

One thing you have to watch when installing a after market cat in a new location is there enough exhaust heat for it to light off. Light off temperatures of most catalytic converters is about 350 degress C, if the carbon is not burnt out, the catalytic converter will plug over time.