Since I bought my '03 SVT (I am second owner, have had the car 2 years), this code is always at least pending if I check OBDII with my scanner. Granted, I do almost all city driving (4 miles between home and office), but my gas mileage is typically only around 17 - 21mpg (20 or 21mpg would be on a tank where I do more freeway driving than usual, which is to say, any freeway driving). I always thought this was a little low, but I drive so little that it wasn't really hitting me in the pocketbook so I didn't pay too much mind to it.
Last winter, I then got another code saying the cat was clogged, and that came after (sure enough) my mpg dipped to around 15. Since I have been fluctuating between 15 - 18mpg.
The car doesn't seem unhappy but the mileage is clearly not correct, and I assume I'm probably suffering a little in terms of performance. Is the first thing to do to replace the O2 sensors, drive it for a tank or two of gas, and see how things shake out?
This car is a stock '03, save for the K&N filter the previous owner put it (drop-in OEM replacement, not a cone or anything). I have cleaned and re-oiled it on my own, followed the instructions, didn't seem like rocket science...