Hyundai Repair: 01 Elantra 02 Sensor, exhaust leaks, catalytic convertor

Question
    I'm looking for some direction on replacing a o2 sensor in my car. I have a cel and diagnostic of P0139 slow response from bank 1 sensor 2. I am planning on replacing the sensor however I would like some insight on the location of the sensor and the process of replacing it. I heard that it is after the catalytic convertor but I'm looking for a more specific answer. Can I get to it from the engine compartment? Is it underneath the car and I need to lift the car to get at it? If I do need to lift the car where would it be? Also what steps do I need to replace it? I heard I need a 22 MM socket but I also need a socket made for the O2 sensor because of the wires. Also do I need to put anti-seaze compound on it? Any help would be appreciated.  

Answer
The sensor mounts in the exhaust pipe just behind the engine.  You may be able to just barely reach it from the top, but it'll be much easier to do from underneath.  To remove it's simply a matter of unplugging it and then unscrewing it from the exhaust.  For the rear sensor on this car, a 22mm (or 7/8") open end wrench is probably the easiest tool to use for removal and tightening.  I cannot speak for the aftermarket sensors, but a factory sensor will come with anti-seize already on the threads.

Before you replace the oxygen sensor, however, check for exhaust leaks.  I've seen the P0139 code arise several times due to a leak in the flex pipe.  And I'm talking about leaks so small that they're difficult to detect with just normal listening.  If you don't hear any leaks, obtain a 2 foot or longer piece of rubber hose 1/4" inside diameter or smaller.  Place one end of the hose in your ear and run the other end around the flexible area (mesh portion) of the front exhaust pipe.  If you find one area of this pipe which has makes a hissing noise through the hose, your pipe is leaking and is likely the cause for the P0139.