O2 sensor replacement question

Stats: 98 4runner, 4 cyl, automatic, 2wd, 150,000 miles.

Question: what kind of mileage impact can the O2 sensor(s)have on the vehicle if one or both are bad. Neither has ever been changed. Nucklehead 1 (toyota service guy) says save your $ - just not necessary to change 'em. Nucklehead 2 (auto parts store guy) says Change 'em - could cause as much as 2 mpg loss or even more.

Also, what other kind of performance losses can they cause if they have gone bad?

Thanks in advance for the input.:banghead:

I don't think they have a substantial decrease in mileage if the front one is bad. Even though you would think so, I didn't notice a difference (although I didn't leave it in for long after I had the trouble code). The rear sensor will not affect performance since it is only a catalytic convertor efficiency indicator.

I wouldn't change them unless you get a DTC (Check Engine Light) indicating one or the other are bad. You may loose some power if the front one is bad. Note that if you receive a trouble code indicating poor catalyst efficiency, it is most likely a bad rear sensor, not the cat convertor.

Yeah, either one being bad can cause a noticeable mpg loss, the EFI isn't running in it's most efficient mode.
But if any are bad it should also cause a error code and trip the engine light.

Big question, what made you take it to the shop to begin with?
If just lower mpg, could be other things.