I've had the car for about 4 years, and no, I have not changed the oxygen sensor. don't know the entire history. when should it be changed?

also, i put my finger up the exhaust to see if there was substantial soot in it, and there wasn't. does that mean its not TOO rich?

Again, guys, I'm looking for some non-techy ways of detecting an abnormally rich mixture, because the fuel consumption is not THAT high - its 18-19 mpg, as compared to 22-23 mpg earlier.

Also, what does the fuel-air mixture screw adjust? someone told me its meaningless to adjust it, since the computer will turn it right back. Then why does it exist at all?

thanks again,
Rags