when to replace O2 sensors??

[714guy]

When do i replace them, i have 99K milies now and eversince i've had the car since 65K i haven't replaced it yet?? I was thinking about doing it b/c my friend did it on his car and he gets a lot better gas milege out of his car. I barley get 21 MPG on the highway.

Which ones do i replace or do i do all of them??? Where can i get them for cheap?? remember starving student budget.

Thanks

[JRE320]

Importec.com is cheap. ;)

[manny]

714 guy

Now would probably be a good time to replace IT.
O2 sensors are considered part of a major tune-up.
Unlike popular belief, they degrade slowly and can be detrimental to your gas mileage, before a Check Engine light is on.
I would replace the one in front of the catalytic converter only.
The one after the cat only monitors the efficiency of the cat, and does not affect mileage in the same manner as the pre-cat sensor. ;)

[714guy]

Originally posted by manny
714 guy

Now would probably be a good time to replace IT.
O2 sensors are considered part of a major tune-up.
Unlike popular belief, they degrade slowly and can be detrimental to your gas mileage, before a Check Engine light is on.
I would replace the one in front of the catalytic converter only.
The one after the cat only monitors the efficiency of the cat, and does not affect mileage in the same manner as the pre-cat sensor. ;)

thanks that just what i wanted to hear.