OK lets see hos fast and good you guys are

There are four wires to the primary 02 Sensor on a 2001 S

One black wire

Two Whites

One gray

I am putting in a AF gauge and need to connect to the sensor wire. Which one is it and how certain are you?

 

Don't look at the wires at the O2 sensor itself. "T" tap into the WHITE wire on pin 16 on the 31 pin connector at the ECU.
Oh, and since I've done it, I'm positive.

 

But does this run from both or just the primary - I have a straight pipe and the mechanical fix to get around the secondary 02

 

The one I stated is for the primary O2 sensor - the one you want for a narrow band a/f gauge. The secondary (sensor for the cat) is located elsewhere.
There are 3 connectors coming out the bottom of the ECU - a 25P, a 31P and a 32P. Find the 31P one and look for the white wire at the pin 16 position (near the central cluster of wires).
I just told you I'm positive. Would you like me to fax you a signed affidavid?

 

Gotcha