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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.
CarloSW2
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