If the worst comes to the worst I've always found the easiest way of unscrewing a rusty sensor is to remove the cat and whatever length of exhaust pipe is necessary, clamp the sensor in a hefty vice, and then use the cat/pipe as a lever to unwind the cat from the sensor.
The trouble with trying to apply leverage to the sensor in situ is that the cat is not rock solid - the engine and exhaust hangers move and absorb all your efforts.
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