My buddy just recently purchased a 1993 civic cx hatch with a d15b8. The car was missing the o2 sensor when he bought it and was smoking like crazy! The car is all stock besides a 4-2-1 header that was on the car. There are two spots on the header for the o2 sensor. One is towards the end of the header right before the cat. The second is right where the 4 are collected into 2. Where is best to put the o2 sensor?! Thanks

 

At the collector.

 

its just a uni header. so if the stock 1 was in the colector put it at the collector. in other cases u can plug in the back. i think thats for the y7 y8s? not sure but the z6 b7s r at the collector.

 

The collector