Hey HT, I have a 99 si b16a2, does the secondary 02 sensor go into the catalytic converter, or is it right after the cat?

 

Plugs into the top of the cat.

 

If you're looking for a replacement, and don't live in CA, you should be fine with one that has the o2 right after the cat. The second sensor doesn't effect performance. It's only checking to see that there are less emissions after the cat.