Should I replace my O2 sensor?

[Secret Chimp]

I don't have a CEL or anything, but I've never gotten quite the mileage I've expected from this car, and it does have 175,xxx miles on the engine. Is it possible for O2 sensors to "wear out" and cause some performance/mileage loss to a certain degree before they trip a CEL? I can pick up a universal Denso sensor for $21, I just don't know if it's worth the effort or not.

[JWC]

What kind of car?

If the sensor was out of range, it would trip the MIL. 

Things that may not trip the MIL...plugs, wires, pcv blockage, air filter.

[Secret Chimp]

What kind of car?

If the sensor was out of range, it would trip the MIL. 

Things that may not trip the MIL...plugs, wires, pcv blockage, air filter.

All of the above are OK. It's a 1997 Accord. I just assume that since this thing was built before Honda went totally nuts for emissions, the range for it might be costing me a mpg or 2 if it's close to being shot.

[JWC]

In '97, all vehicles use OBDII and basically the parameters are consistent.  If it is only a mile per gallon (or two) you're concerned about, I wouldn't go changing emission components in an effort to gain such a small amount of economy.  You might consider changing your driving habits, but not parts.