I dont know if this applies to the exhaust system specifically but when you have two metals of dissimilar composition, they expand and contract at differing rates. Mix in the heat of the exhaust itself causing premature oxidation of the metals and it makes for a tough removal. I had this same problem on the wheel nuts on my Galant. The solution: get out the blowtorch and put some heat on them. Once its nice and heated up, wrench away. It will minimize any chances of snapping the bolt off, as well as the number of curses you will direct towards the car.

 

Mines came right off. No problems whatsoever. And you cant take it to any regular shop like Midas or Meineke or some sort. since they are illegal to have and install. If caught they would be fined up somewhere between $5,000 - $10,000. Thats what every shop pretty much told me. So I had to do it myself. Wasnt had at all. I did have the car lifted though.

 

I did mine myself with no problem. I just put it up pn a jack and gave a good push.

 

I used Valvoline Synpower lubricant, 5 sided half inch drive sockets, and the secret weapon 14 inch pythons Elbow grease people!

 

well i finally got it off like 3 weeks ago. what i was doing was using a 21mm socket (i knew it was too big bu tried for the hell of it) in desperation until i finally went to sears and bought a 19mm socket. i didnt end up wd40 or anything, but i did use the scuff plates of the driverside door w/ all my weight to knock that damn bolt loose. the front was easy once i got the rubber hangers off. but oh my god, i almost ripped my own heart out of my chest trying to get that god forsaken O2 sensor loose. once i got it free (using a crescent wrench and a sledge hammer) i beat the hell out of the stock cat (with the aforementioned sledgehammer) until it bled catalyst all over my driveway. it was a truly bloody day for NCVECS.