O2 sensors

Asked by Nov 10, 2017 at 01:11 PM

Question type: Maintenance & Repair

Hey guys my wifes 2006 toyota carolla le has the
check engine light on and after hooking it up at
autozone I was told it could be the cat or a bad 02
sensor. So the wife went and ordered 1 upstream
and 1 downstream. I have never dealt with these
sensors before so I don't really know exactly what
I'm doing. Lol XD. I just have a hard time
understanding the difference between an upstream
and a downstream sensor besides where its
placed. I heard there was no difference between
the 2 sensors themselves annd it doesn't matter
which one goes where, then again I also heard it is
very critical that it goes in the correct place. What
should I do? Any help would be great, thanks!

2 Answers


the upstream one regulates the air/fuel mix, the downstream checks what is emitted out of the engine


If the part numbers are the same then you can put them in either place. If the numbers are different then make sure you install them in the correct location. You may need a special tool to install them.

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