Heating up the exhaust will help, but may cause you to kill the threads in the bung. But.... get it hot, then use a 02 socket if possible (if not a 21mm is better fit then a 7/8 wrench), try to loosen it up a bit, but once it starts seizing again stop. Spray WD-40 (TAP MAGIC is by far the best stuff in the world for penatrating oil) onto the threads, then tighten it back up. Not too tight though. Then loosen again, spray the threads, tighten....keep doing that. After a few trys, it should come out. We use these sensors at work all the time, but we put anti-seize on them even though Ford puts a tiny speck on them. You should only need to get the sensor/bung warm for the initial break loose.
ST Crew #90