Where is oxegyn sensor located? I have 2000 accent, light says sensor or cat converter.

oxygen sensors(O2S) are located after the cat converters
heated oxygen sensors (HO2S) are located in the exhaust manifold/ exhaust pipe before the cat converters
they are different sensors performing different jobs so rich or lean problems will be the HO2S in the manifold

standing in front of the vehicle looking at the engine bank 1 sensor 2 is the sensor after the cat converter on the bank on your left
bank one is the left cat ---bank 2 is the right cat. Sensor 1 is the one before the cat and sensor 2 is the one after the cat on either side

normally on the outlet end of the cat converter. The heated o2 sensor is prior to the converter.

has 2 O2 sensors has to be replaced poss. #2 after cat. converter

There are two oxygen sensors. Change the other one. Make sure you are installing the correct sensor in the correct location. One is before the cat the other is after the cat. (upstream and downstream)

The monitor is an oxygen sensor mounted aft of the converter. Newer vehicles have an oxygen sensor mounted pre-cat for fuel trim requirements, and a post-cat sensor for converter efficiency.

near the exhaust manifolds, in the exhaust pipes,on both sides.

The check engine light is probably indicating a catalytic converter malfunction the Oxygen sensor that is located after the cat tells the computer if the cat is functioning properly it is possible the wire has been broken or damaged. To see if the cat is there jack up the car and look under the passenger and driver area of the car follow the exhaust pipe from the front back the first thing the pipe comes to should be the cat it looks like a muffler with a wire going to a sensor after toward the back side of it, I doubt they never installed one but I guess it is possible.

You need to check two things.

First, there is a heat shield on top of the front of the motor where the o2 sensor you already replaced is. It's got (4) 12mm bolts. Remove it and check the exhaust manifold (big thing you put the o2 sensor into) for cracks. If there are any cracks, you'll get o2 sensor codes. Replace the manifold.

Second, if the manifold is fine start the car and listen to the exhaust pipe under the engine for leaks. Or if you already know the exhaust is loud... that's gonna do it too. A leaking exhaust pipe on an Accent will set secondary o2 sensor codes.

SHOULD you try to change it yourself, you will need a 7/8" wrench, and a new o2 sensor, or flex pipe, or manifold depending on what's broke. Do NOT touch the tip of the new or old o2 sensor to ANYTHING when you take it out or install it or it will be ruined permanently. Run the car until it's at it's normal operating temperature. Raise the vehicle, and take the o2 sensor out of the car (see below to understand which one) using the 7/8" wrench or if you got fancy and bought an 02 sensor tool... use it.

Replace whatever parts are bad (manifold or pipe) or just replace the sensor you need. Then restart the car and take it to a parts store and have them clear the code. The code (and check engine light) won't return right away, it usually takes a few days. If you did everything right you should be good. If not, it's not a terribly expensive job to have done and by now you should be able to explain to them what's wrong based on what you found when you looked at the car. Just tell them everything you checked and what you saw.

Education: I'm not sure where that half baked answer about o2 sensor 1 being near cylinder one came from but here's the real deal.

Sensor 1 is always BEFORE the catalytic converter
Sensor 2 is always AFTER the catalytic converter

Bank 1 and Bank 2 refer to which SIDE of the exhaust on 6 and 8 cylinder engines. Bank 1 is usually left side, Bank 2 is usually right side.

So... O2 sensor B1S1 refers to the O2 sensor BEFORE the converter on the LEFT side of the motor.

What engine do you have? Does your car have a V6 or a four-cylinder? Regardless of the engine, you will have an O2 sensor after the catalytic converter. On a four cylinder you'll have one in the exhaust manifold as well. On a V6 you'll have one sensor between the engine and cat, and one after each cat as well. The sensor(s) before the cats determine your fuel mixture, and the after-cat sensor(s) are used to monitor the efficiency of the cat.