After about 130 hrs my O2 sensor seemed to be getting a little sluggish; so I decided to replace it. I put a wrench on it and unable to budge it.  A bigger wrench and still no luck – getting worried about cracking the weld around the bung, or some other damage.  Any ideas about getting that thing loose?

I didn’t put anything on the threads when I put it in – perhaps next time; but what?