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Old 10-05-2010, 02:48 PM   #1 (permalink)
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Which O2 sensor do I replace?

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

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Old 10-05-2010, 03:21 PM   #2 (permalink)
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Yes, you just need the upstream sensors to control the fuel. The downstream sensors just let you know if the cat. is working.
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Old 10-05-2010, 03:27 PM   #3 (permalink)
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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?
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Old 10-05-2010, 03:38 PM   #4 (permalink)
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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.

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Old 10-05-2010, 05:21 PM   #5 (permalink)
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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.
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Old 10-05-2010, 05:49 PM   #6 (permalink)
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...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.
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Old 10-06-2010, 01:57 PM   #7 (permalink)
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system does trim on the rear 02 sensor

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 .

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