I did a "Spring tune-up" a couple weeks ago at the request of my CEL and replaced both O2 sensors, TPS, all of the drive-line fluids, as well as the front rotors and pads. I still have to clean the MAS and replace the spark plugs.

My mileage had been hovering between 18-19 mpg until recently when I noticed it begin to drop. Just before the tune-up I was at 15.3 mpg and my first full tank afterwards got me 20.1 mpg! That's mixed driving... one highway trip (about 100 miles round trip) and the rest is city/suburbia.

My CEL was triggered by a "faulty TPS" and "CT O2 sensor slow to respond", but I've heard that the O2 sensors can adversly affect mileage prior to triggering the CEL.

Just FYI...
~Dave