How to replace a 2002 buick rendezvous o2 sensor before CAT. Conv.

You can see, but not reach the before cat sensor from underneath. To get it out, I did it from the top. Remove the 3 ignition coils (2 plug wires each) and ignition control module (this is the part that the coils plug into). You'll have to follow the O2 sensor wire to find it (you'll be working blind). Use the special deep well socket to remove it. There's not a lot of room to turn the wrench and it's difficult to get leverage to turn the sensor out. I put a 3' length of conduit over the handle of my 3/8" ratchet, as there wasn't enough room to get a breaker bar in there. Once you get the old one out it should be smooth sailing. Make sure you follow the directions and use anti-seize. After you get it installed you'll have to clear the code to get the check engine light to turn off. I went to my friendly Advance Auto store and used their scan tool, but I've read that you can also clear it by disconnecting the battery for 5 minutes and turning your headlight switch on.

The O2 sersor will require a special socket to avoid damaging the wiring on the new unit. (the old one really doesn't matter so any socket that works can be used on it)
The first thing to do is spray a thread release agent like PB Blaster® around the threads. You should do it for several days between driving.
Call the auto parts stores to find one that has loaner tools. They will require a deposit until the socket is returned. It will look something like the picture. Yes you will need a socket wrench too.
WORKING UNDERNEATH THE VEHICLE, MAKE SURE ITS BLOCK UP SO IT CAN NOT FALL.
Some more information to consider from a forum

"You might want to take the O2 sensor you just replaced back and get two new ones. Some times new O2 sensors are defective, also when replaceing them I found it better to replace all of them at the same time for sometimes when replaceing only one the computer sometimes sends back false codes. Another thing is if you buy an after market O2 sensor the wires are just a little bit longer and can cause the computer to send back false codes. The best O2 sensors that I have found are the Boch Platinums designed for your vehicle."

You have to use a special oxygen sensor removal tool. It looks like a regular deep well socket but it has a cut out on the side to accomodate the sensor wire. It is made for a 3/8 wratchet. Have you had the car scanned for permanent codes and made sure that the pre cat sensor is malfunctioning? There is more than one oxygen sensor in this car so I would be sure to verify which sensor is bad before going after one which may not be malfunctioning. It is also possible that the Catalytic Converter has gone bad. I have seen them go bad at about 80-90,000 miles.
If you replace the Cat, it will in most cases have the oxygen sensor already attached. A new Catalytic Converter installed will usually cost between $650.00 and $900.00 depending on your choice of a factory part or an aftermarket equivalent. (Most aftermarket Cats do meet or, exceed manufacturers specifications)