Fuel with a high sulphur content will cause this (cheap gas) or an intermittent rich mixture condition (Air filter plugged, WOT and the ECM going into or being in 'open loop' mode).
A good scanner can give you voltage readings of individual upstream O2 sensors and they should rapidly switch from about .1 volt to .9 with the overall average being .45 volts. I have a Davis CarChip E/X datalogger chip that plugs into a OBD II port and can graph the O2 sensors for voltage and switching rate. These can be found on the internet for $50 to $125 and are nice because you can selectivly monitor different things, like intake temperature, engine temp., fuel pressure, voltage, all kinds of sensor readings (TPS, all O2 sensors, 26 different readings in all).
If you are pre-'96 (OBDI), a good scanner will still give you this information, but it is recommended that O2 sensors be replaced at 60,000 miles for OBDI vehicles. For OBDII (some '95's and '96 up), it's supposed to be done at 100,000 miles. Some people go way beyond that with no problems-it just depends on how the O2's have held up over the years,
The smell is just a symptom of a rich mixture and it indicates that the cat is working as it was designed. If it only happens when cold or when running it really hard, it is probably not a major problem-yet. If you are noticing it all the time, you have to find out why it is running rich. If it is left in this condition, it will eventually cause problems with the cat, very expensive and a PITA to replace since they are actually part of the exhaust manifold. If incorrect gap is causing rough idle, or engine miss, it is causing a rich mixture-unburned fuel. A leaking injector, or possibly a bad coil could also cause a problem. Replacing the O2 sensors will not fix another problem that is causing the rich mixture, but it will try to make the ECM compensate for it.
Being new, they will switch quicker (Os sensors get 'lazy' as they get older). I replaced mine about 2 months ago, was getting a CEL and a code that indicated my bank2 upstream sensor was 'lazy'-switching slow. I had no drivability problems, and I replaced both-saw no difference in gas mileage, and it really runs, to me, no different than it did before I replaced them.
Hope this helps.
sweeeeeeeeeet. but any ways i get a very bad sulfer small too while driving. the o2 sensoners are brand new i am running kooks throw high flow cats so i am not sure if it is becuase the cats a to high flow and dont pick all the crap up? or is it possible that my fual air mixture is off even with the new o2? help!