O2 sensor removal technique- loosens 1/4 turn and binds

I'm trying to change the front O2 sensor in my '08 Legacy.  It seemed to be stuck solid so I got a crow's foot O2 sensor wrench... ran the car to get the cat up to temp and then cranked on it. I was super relieved when it began to loosen and then after about a quarter turn it began to bind. I tried repeatedly loosening and tightening it (not many times) It started raining, so I tightened it back down and called it a day. 

Any tips? I'd really like not to screw up the threads in the cat. I was thinking of taking my electric impact driver and using it to do the repeated loosen/tighten thing. Good idea? Bad idea? 

Lots of lube and working it in and out is probably your only choice unless you can get to it with a drill and tap. 

Ugh. I should have left well enough alone. I tried to finesse it, but the threads in the cat came out with the sensor...there's tons of galling. 

 I had a car with an intermittent P0420 code (intermittent enough that I could clear the code,  drive the car in a specific way to set the readiness monitors but not set the code, so it would pass emissions OK)  Thought I'd throw new OEM O2 sensors at the problem to see if it helped. Now I've got an inop car. Trying to decide if it's worth buying a tap and trying to clean up the the 02 bung in the cat or just replacing the whole cat and caboodle. (I'm a dad, I get to make dad jokes, it's the law.) 

I'd chase them and see where it gets you. 

It looks like AZ has a thread chaser in stock. For $10, I'll buy one tomorrow and give it a whirl before swapping out the cat. 

Cats are expensive now, I agree with the advice to try a thread chaser first.

We had the happen on the manifold for my BIL's MR2, and he had a local shop run a tap through it for less than the cost of a tap. 

Tap that cat.

Well, the tap wound up evidently being softer than the bung in the cat. I slowly worked it in, it threads OK, but seems to be getting  significantly smaller in the process. The O2 sensor won't thread in at all. I'm done screwing around, it's cat time. (I also got frustrated and thought that I would try removing the first few threads in the bung with a Dremel tool to see if I could get to the "good" threads. NOPE. )