Upstream and downstream O2 sensors

2007 NISSAN ALTIMA
109,000 MILES • 2.5L • 4 CYL • 2WD • AUTOMATIC
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DUWHOP31
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On my car listed above I have 2 O2 sensors, 1 is located by my radiator (like you can see it when u pop the hood). The second is located under the car where the catalytic converter is. My question is, which 1 is upstream and which 1 is downstream?
May 3, 2021 at 10:05 AM
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JACOBANDNICKOLAS
  • ✓ASE Certified Mechanic
  • 110,194 POSTS
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Hi,

The terms upstream and downstream simply refer to the location of the catalytic converter and where the sensors sit in relationship to it.

Upstream means before or above the converter. Downstream means below or after. In this case, the downstream will be slightly ahead of the catalytic converter under the vehicle.

I attached a pic below. Sorry for the sloppy writing. I hope that helps.

Here is a link you may find helpful:

https://www.2carpros.com/articles/how-to-replace-an-oxygen-sensor

Let me know if you have other questions.

Take care,
Joe

See pic below.
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May 3, 2021 at 8:21 PM
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