I did four sensors today. Huge breaker bars did no good and I couldn't get an impact on them. The next best thing was smacking a wrench repeatedly with a hammer. All of them broke loose pretty quickly with that method.
Assuming you aren't using a lift and you're struggling with the sensor socket and pb blaster, unplug the sensor, remove the cat pipe and work on it off the car where you can get some decent leverage.
yeah, like sheen stated. Take pipe out of car, then attempt to remove sensor. soak in pb blaster, heat with torch, remove. either use an O2 sensor socket and rachet or use a 7/8" combination wrench. tap on tool until it breaks the sensor free from the pipe.