Hello, Was wondering what the upstream and down stream o2 sensor readings should be reading on my scanner?

I have a 2002 Grand Prix GT 3.8L

I can't see to find that information on line, or my wording is not catching that.

Any help on what they should be, thank you.

Joanne

The pre O2 should be constantly changing above/below 450mv.......the post O2 sensor should almost be flatlined, or changing very slowly(this depends on when the catalyst stores/uses oxygen).....

When the post "mirrors" the pre, in other words, rapidly changes in unison with each other, then a P0420 is set....this is usually an indication of a bad cat.....

red is the pre, green is the post