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Old 01-24-2015, 07:25 PM   #1
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Default What should the o2 sensor readings be...

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
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Old 01-25-2015, 12:29 PM   #2
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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

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