Upstream and downstream O2 sensors

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.