Recently I wrote: "This vehicle is a F250, 1989, 351 engine. The problem started with a bad fuel pressure regulator. It made the truck run very rich for several months. I replaced it as well as the O2 sensor. Now, the truck runs less rich but still puts out some soot and black smoke. I checked the recently replaced O2 sensor, it was totally covered with soot. My question is this: Is it possible that the cat (18000 miles)
has been destroyed by the rich running conditions and needs replacement? Would that affect the rich condition?
Also I get a code 41 (lean) which of course flies in the face of what is really happening.
Other than that the truck runs great and uses very little oil. If it were not for the smog test, I would not do anything."

And boy, did it ever put out some ugly wet black soot! After posting here, I went back to the O2 sensor. I took my propane torch and burned off the soot that covered the sensor. I reinstalled it. Then I disconnected the battery for 10 minutes to empty the computer of all codes including the ones it might have "learned" itself. After starting the engine I drove full blast(within legal limits, of course) for about 15 miles. At first, some soot blew out. After a while that stopped, even at full power. Now I have been driving for a couple of weeks. No soot, no rich condition, no code 41. Apparently the cat works now, as does the O2 sensor.
I thought you would like to know.