It does not. Only AZB engined 551G box facelift S4s have rear O2 plugs.

I'm having some trouble getting this to work properly.  When I log for AFR, I get goofy values that range from 9.6 up to ~12.1.  After installing the Zt-2, I confirmed that I'm getting ~2.5v on the white wire.  I also logged with the white wire disconnected and the value was consistently ~10.5. (Should that be right?)

Based on the above info, here's what I did:

Ran the white wire to the unplugged green O2 sensor connector (passenger side firewall)

Connected wire to the signal line, which is the female connector separate from the other three

Edited ME7L ecu file and added the following line:
;Name,    {Alias},                        Address,  Size, Bitmask, {Unit},        S, I,            A,     B, Comment
uushk_w, {O2SVoltageSensorWB}, 0x381116,    2,  0x0000, {WB-AFR} , 0, 0, 0.009766, -9.6, {}
 
Edited ME7L cfg file and added the following line:
;Name      [Alias]                     
uushk_w   AFR-WideBand

Any thoughts on where I goofed this up?

I'm running an M box build. Is that what you mean?  I'm still pretty new to this side of things so please point me in the right direction.

The value that gets logged changes based on whether or not the white wire is connected. I took that as a good sign that I was logging in the right place. Obviously I have something goofed up though because the values just aren't right.

I should be all good here.  I have a 2010 model Zt-2 that was bought through the jfonz group buy last summer.  The AFR graph in the ZT software shows normal values.  Also, the output of the white wire is ~2.5v as it idles.

Assuming that the Zt-2 hardware is good up to the green connector, I'm either logging the wrong value or the signal is getting goofed up some other way. 

One thing that seems strange: The signal wire in the green connector is already at close to +5v just hanging open without the WB output attached.  Is that normal? 

Nope...the variable we are using is voltage from the sensor before any ECU conditioning.

Can you explain the problem better?

You get stoich readings at idle, but strange readings during enrichment?

Have you compared ME7L logs to ZDL logs?

Have you tried using bank 2?

Arrgh!  It was totally my bad. 

I went back through everything from square one and I realized that my ME7L cfg file was referencing a different ecu file than the one I was tweaking.

In the earlier version of the ecu file, I had entered the uushk_w variable but only for one byte, not two. That's why there was some effect, but the values were way off.

Once that was cleaned up, everything worked as it should.

Good stuff, now on to some WB logging!

Thanks for everyone's help.

Closed loop won't be smooth just because of the nature of narrowband lambda control.

WOT should be fairly smooth.