I have a 03 Tahoe with 5.3. I've been chasing a rough idle in gear issue and been looking at my O2's.. I have no codes. both banks pre cat's are switching from a value less then 100 to up 700--800ish.. the post cats are staying at a high (Around 800ish).. This isnt right??? they should switch correctly???

 

That's normal.

You need a graph logger to accurately show O2S switching. The fronts are supposed to switch rapidly and the rears are supposed to switch slowly. The minimum ratio is 10 front / 3 rear. If the rear were to be switching at the same speed as the fronts, that indicates a faulty catalytic converter.

100mv indicates lean cycle and 800mv indicates rich cycle. If the average is around 450mv on the front sensors that's target AFR I think.

 

If I recall correctly the stock tunes do center on 450mv, above operation sounds normal to me. Front 02 switch rapidly unless you are WO and in Open loop then stay go steady around 880-920 or so depending on how rich the tune is. At WO they are not accurate anyway so....

 

Thanks for the replies. i will put my brother inlaws snap on tool back on it and graph it this time.. Looking at real time data (numeric only) it did show the pre cats switching but the post cats where not, they where a high value only...