Originally Posted by benzfan

Out of curiosity, have you noticed a drop in the full throttle power to go along with the drop in fuel economy? If so, your cat could be at fault (partially plugged).
I once had a Honda that melted it's cat, giving me a lack of highway power and 15mpg. 2 dealers gave me idiotic diagnoses of 1) failed ignitor in the distributor and 2) plugged fuel filter. It finally got so bad that I was sick of dealers doubting my plugged exhaust theory that I pulled out one of my ice axes for the next service manager and bashed some holes in my muffler. The car remained silent and the service manager believed me.
What was remarkable though was just how far the fuel mileage dropped before the lack of power was felt. If you're playing underneath with the O2 sensor anyway, it couldn't hurt to take a peek at the cat internals.