O2 sensors before or after the cats?

[motorholmes]

I fabbed my exhaust a couple years ago and I don't know why, but I put the o2 sensors downstream. I'm currently reading, reading, and reading some more trying to sort out a couple issues, and I just wondered if one of my problems is that I need to put the o2's upstream instead?

[Frenchy]

Yes they need to be ahead of the cats.

[motorholmes]

I have narrow band, BTW

Wow, so that's gonna totally goof up any tuning I do? That sucks!

[motorholmes]

What about these vehicles that have o2s before and after the cats? What are the ones after supposed to do that's different from the upstream ones?

[kerrige]

The after cats are to make sure the cats are doing their job the before cats are to get your afr closer to stoich.

[motorholmes]

I looked at the Haynes manual and it shows one pre- and one post- converter. I don't remember what they were when I tore out the exhaust system.
Is it possible one of my sensors is pre and one is post?

Boy I'd feel pretty stupid if there's 4 o2s on my truck and I just didn't see the other 2. Hey, it's kinda low to the ground and hard to slide under there If that's the case, sorry I wasted your time with this

[Frenchy]

There is 4. First 2 are close to the manifolds near the top the y-pipe. These are the ones the PCM uses for fueling. Then there are two after the cats whose sole purpose are to measure the catalysts effiencey. That is there whole purpose. So if you don't run cats you can effectively delete the 2 rearmost sensors with no ill effects