Where is the oxygen sensors located in a 2002 Nissan sentra 1.8l?

Catalytic converter I think

The 02 Sentra doesn't have a "regular" catalytic converter, its an all in one manifold converter assembly.

*There are 4, they are located in the exhaust manifold. On the 02 Sentra 1.8 there are two wiring harnesses that have blue plugs and two that have green. Those are the harnesses for the Oxygen sensors. Follow the harness down the manifold and that's where they are located.