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Try warming up the exhaust system for 2-3 minutes, and wrench it again. If necessary, add an extension to your wrench for additional leverage, being careful not to apply too much pressure that you snap the sensor off. O2 sensors are pre-coated with an anti-seize compound, so you should be able to break it loose.
Once it starts to move, work it in and out. Go out a little farther each time, while spraying with PB, it will loosen up. If you just try to take it out and rust/crud is in the threads it will just keep getting tighter till it breaks.
They make a nice O2 tool that works with a 3/8 drive. I put the tool on the right side O2 and used several long extensions from the top to remove it. Worked slick.
I bought the socket for $15 bucks and it just slips. Now its vice grips time and new O2 time. The sensor works but to use the WB O2 at the the dyno, it has to be able to come out. I'll try the PB Blaster.