Almost finished installing the turbo back system on my N. Just some minor adjustments needed and heat shields to be put back. However, the downpipe from afe has two o2 sockets before the catalytic converter and there's only one o2 sensor for them. Which socket is the sensor supposed to go to? And what about the other hole? Should it be plugged or unplugged? Nothing mentioned about the two holes in the instructions. That's no plug of sorts that came with the downpipe either nor is it listed in the parts list.