There are 4 oem o2 sensors, 2 on the downpipes/precats and 2 more on the testpipe section. Depending on your exhaust setup, some aftermarket downpipes and testpipes have bungs welded on for the o2 sensors. From what ive heard you only need the front 2 sensors connected on the downpipe. I have the 2 on the testpipes disconnected and it runs fine. Also check for exhaust leaks because you shouldnt get any fumes in the cabin.