1999, Cav. 2.2, O2 sensor behind the cat. converter has crapped and the parts people asked primary or secondary O2 sensor.
I don'k know. anyone tell me which is which, the one on the engine man. or the one behind the cat. conv. under the car.

primary = before cat
secondary = after cat

the one on the engine man.
I believe that's the header, just for future reference.

technically, it's in the flange that connects the downpipe TO the headeR lol.

right but the 02 sensor's socket located in the header

If you use a code reader, Bank 1 is before cat, Bank 2 is after. Do not replace the Primary O2 sensor with a Bosch one. They are notorious for crapping out and failing. I've returned 3 of them for our cars in my store. Single wire ones from the dealership are around $65 bucks. Get a delco replacement for the primary, secondary you can use bosch.