Which O2 sensor do I replace?

[KCF3712]

Hey Ya'll. I'm wanting to replace my oxygen sensor(s) and need some advice. I have a 2007 Kia Spectra with 108K miles. It is recommended to change them at 100K. To save a little $$$, would it be OK to replace just the upstream or should I replace both up and downstream for optimal FE? In my research, it's the upstream that regulates the A/F mix, right?

Thanks in advance

KCF3712

[hypermiler01]

Yes, you just need the upstream sensors to control the fuel. The downstream sensors just let you know if the cat. is working.

[Daox]

I've never replaced an O2 sensor I haven't had a problem with. I know I've heard this can give you bad mileage, but how many here have replaced them earlier and seen good results?

[cfg83]

KCF3712 -

I think that both at once are preferable, but hypermiler01 is correct, the upstream (pre-cat) O2 sensor is more integral to closed-loop operation. Also, for your Kia Spectra, the OEM brand is usually preferable. For instance, in my drivetrain, the OEM was Denso. When I switched to a Bosch sensor, it would raise error codes more frequently. I was told at saturnfans.com that the Bosch sensor is more "sluggish" in it's response rate, so the ECU/PCM thinks there is something wrong, so it raises a false error code.

CarloSW2

[cfguy2000]

If you aren't suffering from bad gas mileage, you are probably better off waiting until one fails then to just replace it for no reason. The cars computer does a decent job of knowing if they are bad. Plus O2 sensors are pretty expensive. I know because my gas mileage dropped off significantly about a month before my check engine light came on for the bad O2 sensor.

[Old Tele man]

...how "close" are you to having to having a required emissions test done?

...if you're close, do it now. if not, wait.

...O2-sensors can run from $60-$200, depending on what you buy, aftermarket or OEM replacement..and, unfortunately, their 'quality' varies as much as their prices.

[mwebb]

you would have to look in scan data to confirm this
most cars manufatcured after 2003 DO trim on the rear 02 sensor.

you can look in the Mode 6 test results
look at the test limits and the test result - test results should not be anywhere near test limits

if you have a scope
the 02 sensor should switch in 100ms or less , that is 10 times in 1 second
at 2k rpm hot with no load
must cycle from 150mv to 850 mv in 100ms or less and back in 100ms or less

does it ?

is it an AFR sensor or an 02 sensor ? how many wires ?

best answer is
if it aint broke - DONT FIX IT .