Did a search but didnt find anything, I believe there are 4 02 sensors? two before flow and 2 after flow, is there a way to determine which is bad? aftermarket cost is $130.00 ea

 

it's normally a good idea to replace all of them- at the very least replace both on whichever cylinder bank is bad.

 

Geez one more good reason to have the 4 banger.

MB recommends replacement every 100K miles.
The fronts ones actually effect the fuel mixture, mileage etc.
The rear ones are only there for comparison to the front ones to
supposedly inform you if your cat is not performing well.
Mine was clogged and didn't result in a CEL.

I replaced only the front one for the sake of mileage (and the fuel ratio, which can burn up your cat), so far ok.
If I start getting CEL's for the cat being bad I'll replace the rear.
(Since the cat is already replaced)
But oddly enough, thats the expensive one, almost 200 bucks even on discount.