when to replace O2 sensor?..

I have a 97 escort, 60K miles right now. I'm a mpg snob, and i would like to do everything reasonable to keep her gas consumption down. reading this forum way back, when i had an older model, i read a number of recommendations that one switch out an old O2 sensor. however, the factory manual, the mechanic at the (sketchy) inspection place, and an autozone dude all think you shoudln't do it unless you get the oxygen sensor error code.

so- what's the word on the street? just wait until i get the check engine light? do it after six years, no matter how many miles? pull it out and check it and replace it if it (insert some diagnostic principle here)?

thanks,
anthony

Most O2 sensors will last more than 100K miles without causing a decrease in fuel mileage. I suggest that you leave it alone until you get closer to the 100K mark or you find that the fuel mileage has decreased significantly. Keeping good plugs, a clean air filter, properly inflated tires and alignment and keeping you foot out of the gas will do more than replacing the O2 sensor at this point.

155,000 miles on 97 Escort without an oxygen sensor change. Still gets 43 on the highway and 40 in town.