Are all O2 Sensors created equal?

[SJ&I Santoliquido]

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Hey all;
I have a question about the O2 sensors on my 1996 A6 Wagon. It has 4 (how d=
id they figure out that 2 wasn't enogh, but 6 would be too many?). The two =
forward ones (close to the engine) are three-wire sensors. The two behind t=
he cats are four wire sensors.
Rod at TPC said the three-wire didn't show up as cross-referenced to the Bo=
sch 13913 (universal) in the Bosch catalog. And he didn't know a universal =
for the four wire.
So I have three questions:
1) Any BTDT using the 13913 in the front locations (or other inexpensive op=
tion).
2) Any inexpensive substitutions for the rear, four-wire ones?
3) What are the  rear ones there for anyway???
4) I thought all O2 sensors were created equal. That is: w/ the exception o=
f the end connector aren't all three-wires the same etc.???
That's four questions isn't it.
TIA
Steve
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