help with 02 sensor location

1996 Chevy Blazer 4.3 2wd....

Looking under the car to locate the sensors, I see four of them. My scanner says: B1-S1, B1-S2, B1-S3, and B2-S1. I figure the last one,
B2-S1 is after the catalytic converter, but the others; I am not sure of .

This engine has a duel exhaust coming out of it and one sensor in each of the pipes. Where the duel exhaust meets the single pipe (before the CC) there is another sensor.

How do I know which is which?

Bank one is the cylinder bank that houses cylinder #1

Bank two is any -other- bank the engine might have (as would be the case in a "V" type engine)

The 4.3 liter engine in your S-10 uses a rather unique O2 sensor arrangement, there are three pre-cat O2 sensors and one post-cat O2 sensor (go ahead, look).

Sensors are numbered front to back respective of their bank location, any sensor fed by both banks defaults to bank one (as is the case with your B1S2). This is standard SAE nomenclature for OBD2. So, the O2 at the outlet of the exhaust manifold for cylinders 1, 3, 5 is B1S1, the O2 at the outlet of the exhaust manifold for cylinders 2, 4, 6 is
B2S1, the O2 after the "Y" in the exhaust, since it is fed by bank one becomes B1S2, the O2 after the cat, since it is fed by bank one becomes B1S3. Once the exhaust pipes join together at the "Y" bank two is no longer an identifier.

The person posting the conflicting information probably doesn't do this stuff for a living, doesn't have OEM training, probably isn't an ASE certified master technician w/L1, etc.

Easy enough to verify, just plug in your favorite scan tool and disconnect any sensor in question, observe on the scan tool which O2 sensor stays at threshold voltage and note the displayed identity of said sensor.

There are some Chrysler products that are exceptions to the above rules, but then, a professional would be properly equiped training wise to handle such anomalies.

I am getting conflicting information on you set up below.

Someone else said that the right side of the Y is S2 and after the Cat is S2.

I am confused again as I cannot find any information of this topic anywhere.

B1S1= driver side upstream
B2S1= passenger side upstream
B1S2= after Y, ahead of cat-con
B1S3= post cat-con

Thanks for taking your precious time to explain to me, I will study this further, what you previously said. So, on the Y anything on the passenger side of the Y is B2, below the Y joint is B1.

Thanks much...

Thank you.... that's exactly what I needed to know. (My original guess was wrong.

In the case of your truck, yes.