2001 Toyota Corolla OXYGEN SENSOR

There are at least two, one upstream and one downstream.
They are in the exhaust system. Bungs, welded to the exhaust pipes hold them in place, they screw in to the threaded bung.

The upstream is before the convertor, and the downstream is located after the convertor.

You can rent O2 sensor sockets to remove them from most autoparts stores.

Price is generally anywhere from $50-100 each.

Use antisieze compound on the O2 threads when you install new ones.