Downstream O2 sensor - Does it affect fuel mileage?

[kcackler]

I've had the check engine light on for awhile. I finally checked it today and I've got two codes about the driver side downstream sensor. Chances are it's toast, which makes sense at 110,000 miles.

My question is - How important is it to replace this sensor? Since it's downstream, I would think that it might not affect fuel mileage, but my internet research is leading to conflicting information.

I can get an OEM replacement for about $70, but the question is, would you?

This is on a 2001 SE.

[atraudes]

I replaced mine when the CEL came on for them. Having the light off, passing smog with no problems, and knowing if something new comes up all made it worthwhile. That said, everything I've seen says that the downstreams have absolutely no bearing on engine dynamics. They're there to babysit the catalytic converter's efficiency. So, I'd recommend doing it, but it certainly doesn't need to be the top of your todo list.

[kcackler]

Kind of my thoughts as well. I don't like seeing the CEL every time I turn on the truck, so I want to replace them, but at the same time there are other, more important places for the money to go right now.

[donald]

I went about 4years with my bad downstream O2 sensor. I even did my novice-MPG trials with with the bad O2sensor.
I don't recall the CEL code, but it was the passenger side (ie. right side from driver seat POV) for "High Voltage error" or something along those lines.

I was able to "get away" with it during my bi-annual smog check, since one day it turned off and I rushed my truck in to get it smogged

I finally changed it out upon the 2nd time around for smog for about the same price as you mention ($70 from online store).

From my experience, there was no ill effect to engine performance.