For starters i have a 95 del sol with a d16y8 swap in it. I recently got a new exhaust from the header back. My stock exhaust manifold had on 02 sensor threaded into the top part near the head where the 4 pipes come down. The kicker hear is it was never plugged in to anything. (it was threaded into manifold but where and plug just hung freely) No CEL. My new header is a 2 peice 4-2-1 and has the 02 sensor bung in the same spot as the stock manifold. There is also one a hole for a sensor on the down pipe (2nd peice of header before the cat. I have a universal high flow cat welded ( doesnt have a 02 sensor hole) For now i have both holes plugged with 02 sensor plug bolts. Now it seems like my car hesitates or bucks at low rpm. Will not running 02 sensors cause this? Like i said the 02 sensor was screwed into stock manifold but not plugged in to anything there for i just capped it off on the new header. But now its running funny. That and will it cause me to fail emissons? Another question is would i need to use an 02 sensors in both holes?