What the O2 sensor is telling you is that the exhaust is much too rich with unburned gasoline, that is why you are getting the sulphur smell. Most likely a leak somewhere and not the O2 sensor.

Start replacing vacuum lines, they are old and brittle. Remove the throttle cover and inspect the vacuum tree. If you have EGR, clean out the EGR valve. Remove and clean the spark plugs, check the gap or replace the 3 prong plugs (NA engine) or copper plugs (turbo).

Check the air hose from the throttle body to the air cleaner for holes.

If there are leaks before the CAT, you might want to consider replacing the down pipe.

But, vacuum lines first.

Klaus
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Always willing to listen, just not able to take direction.