By the collector,right after thr y-pipe or behind the cat?

 

Within 6' of the engine.

You don't want to be after the cat..
Also depends on where it was in oem form.

The ECM in some applications is very fussy about how long it takes to see a change from the O2 when it toggles the BL to test the O2. You *can* have false code 13s, 44s, and or 45s from being too far off in any one given situations. TBIs are the most forgiving.

 

I guess I'll put it right on the header collector.That is where all the headers I've seen had it.Mine don't have a spot so I'm going to weld a bung in.I just though a heated o2 woud be better back off a bit from the heads.Possibly getting more consistent o2 readings,not getting skewed by heat.Also the O2 sensor, right at the y-pipe would get readings from both heads.

 

Currently, I have mine at the passenger side collector on the pipe side. I bought Dynomax reducers (from 3" to 2.5") that have a bung in them already. My goal was to have an 02 sensor in each pipe to measure the difference between the banks, but I just have a plug in the driver's side one for now. I have a Flowmaster y-piece that joins two 2.5" pipes into a single 3" (no cat) and I should relocated my sensor in the future to that location. It sits in the same spot that the stock cat was. BTW, I have shorty Hedman headers (1 5/8")