Almost there...

I'd keep at it with the penetrant, and maybe heat up the fitting on the cat. Clean up most of the penetrant first. Be careful with the torch!!

Have you tried a breaker bar yet?? (I wouldn't get too caveman on it; don't want to torque the headers too much, as has been previously mentioned.)

I have used a freeze spray in the past (I think I got it at Radio Shack, but it was a few years ago). Cooling down one part (O2 sensor) and heating the other (cat fitting) sometimes will do the trick.

Good luck . . . Jimbo

PS - 1-4 is on the right, (passenger's side)

 

It sounds kinda rough, but if you can get an impact on it rather than just plain torque that will likely help. In its rawest form this can mean putting a ring spanner of appropriate dimension (19mm?) on it and then giving the spanner a lateral whack with a hammer.... These are often tough to remove because of the corrosion created by the continuous heat cycling - keep at it with the penetrant and try the judicious impact approach.

 

Couldn't you simply disconnect the O2 sensor (under the airbox, I think), then pull the cat (and sensor) off the car? Once the cat is off with the o2 sensor dangling behind, you can work more securely on a workbench.

 

Definitely!! That would be my next suggestion if the penetrant/torque/heat/freeze methods don't work. If it still doesn't budge with the cat off, hey, you're halfway there to putting on some Hyperflows!!

To be honest, I was sorta hoping my sensors were frozen solid to the cats so that I could somewhat justify buying the Hyperflows. Course, once I'd gotten the cats off, I'd have to spring for a new exhaust, since the hard work is already done!

PS - Sensor is 22mm. You would have to cut the connectors off to get the box-end over them. At that point, you're committed.

 

I used a crow's foot with a long extension and breakerbar.