There are going to be atleast two. One for sure will be after the convertor. Its sole purpose is to monitor the cat and let you know if the cat is not working. The computer does NOT use this O2 for anything else and does not make adjustments based on the reading. You will have atleast on more between the engine and the cat and that is the important one. Some vehicles have one on each bank of cyls. but as I recall you only have one beside the transmission. It is very important not to get a cheap O2 sensor as this is the single most important computer sensor. I prefer Bosch if possible. There is a special extra deep slotted socket that is used to take it out but as long as you can get a wrench on it you should be able to spin it out. I have replaced about 100 over the last five years in different customer cars and have never had one not come out easily as long as I have had room to move. Make sure to unplug it first. A lot of the time you will need the number off the old one or the color of the connector to get a new one. I also recommend disconnecting the battery for at least 10 min if not longer before starting so the computer will relearn faster. Good Luck. By the way I am an ASE Master Tech.
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