O2 sensor or cat?

Ok now my gas mileage has become about 2 to 3mpg worse and the exhaust smells rich. But I am not shooting up a code/no service engine light. Now I do have a little over 100k now and I know the O2 sensors are said to be good for 100k but not having a service engine light on makes me wonder if it is an O2 sensor. What do you guys thinks?

If your car thought it was having an air/fuel problem, you would get a check engine. The O2 sensors are designed to give feedback to the PCM, both before it goes through the cat, and after. If it were a problem after the cat, you would get the P0420 code. If you haven't popped, I wouldnt worry about it.

Mileage fixes?

New plugs.
New Fuel Filter.
Injector cleaning.

=)

-Keegan

Ya i have new plugs, newer filter, and always use cleaner.

Well then you may end up dumping 60 bucks for the rear O2 sensor, and another 35 for the front =(

Do you think I should just keep waiting for the code to go off?

Personally? I'm cheap. So yes. Mine pops the P0420 all the time, but with my aftermarket cat, it's to be expected. Another option is to buy an O2 Simulator... but that might just plug the hole, instead of fixing it..

http://www.zzperformance.com/grand_prix/products1.php?id=179&catid=107

But seeing as how you ARE over 100K, it MIGHT be time for a new Cat. But don't get the stock one again... go for aftermarket, mainly because stockers cost somewhere around 300 bucks to my understanding... you can get an aftermarket for much cheaper...

my take on it is that youre worring too much....

the downstream o2 monitors the cat, the cat burns off any fuel the engine doesnt burn AFTER its up to temperature.

stick your nose by the tailpipe when the engine is cold (therefire getting more fuel) and before the cat is hot enough to burn off any left over fuel and it would make sense for it to smell rich.

as for mileage, mines gone down too, happens every winter.  engines use more fuel when they are cold. the colder it is the longer the engine has to run before its up to temperture, so its not as efficient as it is in the summer.  and then if you warm up youre car ever, even just to defrost the windshield, thats going to hurt it more too.

touche. perhaps i am just looking for an excuse to buy a high flow cat lol

Don't just jump at the O2 Sim yet.  If you have a 2004 Monte or newer they are useless.  The new O2 sensors measure the exhaust heat as well and the sims don't crack that code per Zoomer.

The only car I've ever had to mess with an O2 sensor was my 2.2L beretta.  And trust me...  an O2 sensor does wonders.  The engine will run better, and get better mileage.  They're always the first things I do if having mileage problems (besides air filter)  Really the 60$ isn't horrible for what it does. 

What year is it? Check this out http://www.montecarloforum.com/m_12459/tm.htm , yours may qualify, mine did. Mine was throwing a po420 code, but I don't think it is absolutely neccessary, maybe it hasn't gotten bad enough yet to throw it.
I called GM direct about it and they got in touch with the dealership here. My vin# fell right in the middle of the bad ones.