I finally got around to changing my spark plugs today. The plug tips for cylinders 1-3 were completly black, but dry. The plugs for cylinders 4-6 were not blackened at all and seemed show normal wear. I think the plugs were about 40 to 50 thousand miles old.
Earlier this year I had a bad oxygen sensor that I didn't get around to fixing for a few months. I don't remember which one threw the code, but I eventually replaced all four of them.
Could riding around with a bad oxygen sensor have caused the fouling that I see with those plugs?
If not, what else could be affecting cylinders 1-3, but not 4-6?
To answer your original question here, yes. A bad O2 sensor is exactly what would throw off the A/F ratio on those three cylinders, causing the plugs to foul.
Disconnect your battery for an hour or so to reset the computer. The Jeep has been running with three fouled plugs and the computer has been compensating for that over time. Resetting the computer will erase those adjusted parameters and go back to original programming, so you won't get uneven plug wear or other issues from computer compensation.
When you hook the battery back up and start it, turn the key to on, not run, for three seconds to prime the fuel system, then turn it to start. Don't touch the gas pedal until the Jeep has idled for at least ten seconds. The computer learns all of its long term idle parameters from that first start up, and pressing the gas to get the engine to turn over badly skews the programming, usually causing stalls or rough idle.
I would pull the plugs again in a few thousand miles just to make sure they all appear consistent and aren't fouling.