Got my O2 sensors mixed up and need to know which is which. I looked up the part numbers and still don't know which is which. Plugs are the same. Both have 4 wires of same color.

I also looked up by VIN and the numbers don't match. They are from a single CAT Federal emissions 99 4Runner 3.4.

The one on the left with the longer pig tail and 3 hole rows is Part# 89465-35540. Has carbon build up so if I had to guess goes in front?

The one on right with short pig tail and 4 hole rows is Part# 89465-35380.
It's clean so I was thinking rear after the cat?

Anybody know for sure?

 

Well I was hoping somebody would have some info for me.

I stuck the one with the carbon build up in the front and the clean one after the cat. So far no CEL but, I haven't driven it yet.

 

I'd have installed them the way you did. If a problem pops up switch positions provided the wiring reaches.