On my 89 s15 Jimmy 4.3 2WD Auto I had a code for the O2 sensor. A normally 30 or 40 minutes job turned into a much larger job.
The O2 sensor would not budge and I ended up rounding the Hex base. I cut the body off the sensor in order to get a socket on the sensor and that did not work at all. I bit the bullet and removed the Y-Pipe and of course a couple of the Y-Pipe studs unthreaded from the exhaust manifolds instead of the nuts. I first tried heating up the remaining part of the O2 sensor and could not budge the sensor. I resigned myself to drilling out the sensor body and hopefully not damaging the threads. I started drilling out the body and after a few bits of larger size I gave the torch and pipe wrench one more try. The remainder of the broken sensor threaded out.

I had to get a set of exhaust gaskets, Exhaustt Studs with nuts & springs and install them on the exhaust manifolds. I had to get the nuts off of the studs that remained on the Y-Pipe., I chased the threads for the O2 sensor (coated the threads with anti-seize), reinstall the Y-Pipe, Install new O2 sensor, install Cat and attached the Muffler with the tail pipe assy. Hopefully no exhaust leaks. Fortunately, no leaks and no codes.

Note: Previously I got rid of all U-Bolts for the exhaust and installed band clamps that do not crush the exhaust pipes and makes it much easier to work on the exhaust system.