How to identify O2 sensor wires? How to tap them? How to read them?
 

I'd like to tap an O2 sensor's wires and hopefully use a DMM to get a relative idea of A/F ratio while driving. I'm pretty sure mine are narrowband.

The normally sensible engineers at GM decided that my pre- and post-cat O2 sensors should have the same color wires and be in the same bundles. Besides physically tracing the wires or using a tone generator & probe, how do I tell them apart?

Once I find them, I assume I can just carefully tap them with a scotch-lok quick splice. I won't need to cut and interrupt them to put my meter in series, right? I don't want to interfere by measuring...

Once I have my DMM hooked up, I'm pretty sure I'll have to look at voltage. Would a higher reading mean richer or leaner? A little googling makes me think that higher voltage means richer ratio.

Yes- a higher voltage (above 0.45v) usually means rich.

Thanks, that's one question answered. :)

Also, they have three or four wires (I'm working off the ECM pinout here, I was under the truck looking at them earlier but I don't want to get dirty now) each. One is described as "low" and the other "high", and a third is the heater, and a fourth is "low reference". Which do I probe, the low or the high? Or, do all these wires mean I have wideband sensors (and then what does that mean I can additionally do)?

heated O2 sensors are not wide-band O2 sensors. if you tap into the O2 sensor, it's going to give a signal that flips rich/lean several times a second... very hard to read. I've known people that put a A/F gauge on a regular O2 sensor and it was pretty much useless except as a flashing light. if you want a readable gauge, get a wideband O2 sensor and gauge.

Drat. Wideband sensors are expensive.

There is a LED bar graph DIY gauge that I found after I posted this, which appears to be very popular, but I need to find a little more info on building it or just suck it up and buy one.
https://autospeed.com/cms/A_0217/arti...popularArticle

So, what do "low" and "high" mean?

What kamesama said. I think without a wideband sensor you won't be getting useful information.