I put in a new O2 sensor two weeks ago. I noticed a slight increase in mileage. In my second tankful, I noticed a drop in mileage. I thought it was from driving into a headwind, but yesterday it was still there with no headwind. I switched to the diagnostic mode and saw my new O2 sensor was reading 0.00 to 0.01 at 60 mph. Below 40 mph it would fluctuate with normal readings. I looked around and checked connections to see why it would be running full rich. ( I deducted it was full rich because if the O2 was reading that lean, my mileage would read fantastic, but it was only 20 mpg. To shorten the story, I had kept the old O2 sensor in the trunk, so with some burned fingers, I put the old one in... Mileage and O2 readings back to normal! I will return the "new" sensor while still under warranty on Monday. I doubt that would have been caught that quickly with any other car than the REATTA!!!