Oxygen sensor location?

Hello, Bank 1 is the side of the engine that Cylinder #1 is on, I will look up the firing order and service info for you and post it here. So Bank 1 Sensor 1 is the front oxygen sensor and Bank 1 Sensor 2 is on that same Bank of the engine (1) but the Sensor 2 is behind the Catalytic converter. Ill grab you the wiring diagrams as well so you can identify which sensor you are looking at by its wiring coloring. As for the Bank 1 Sensor 3, that sounds like a scan tool glitch. there is no #3. In some cases on older vehicles they had only one oxygen sensor after the Cat monitoring both banks. But you said you counted 4, and I see 4 on the wiring diagrams right now. Give me a min and ill post this for you.

Do you need any other information on the vehicle?

Ok I have the oem and the aftermarket wiring diagrams pulled up for the Oxygen sensors, and that Bank 1 Sensor 3 is a misprint(error), its common to find little errors like this on older service info, so backing it up by checking the aftermarket ones is the best practice. Let me just make sure we know what that mislabelled sensor actually is.

This is a semi unique system, it looks like they have an oxygen sensor for each exhaust manifold which is probably what the ECM is using to monitor and control the fuel injector's pulse width, and those Pre & Post 02s are obviously monitoring the Cat and Ill bet the ECM will calculate the Long Term fuel trim (or GM's Block Learn) according to those sensor's readings. I would expect the last sensor in the line there to have a pretty steady reading, whereas the right and left front 02s should be swinging between 0.1-0.9volts roughly at about 1Hz per swing. Let us know what you find and what you're dealing with.

I wanted to add this to help direct you right to the correct sensor.. Hopefully this helps some.

I put 2 brand new sensors in bank 2 sensor 1 positions and the trouble codes came back. I put an old sensor in I had laying around and the code hasn't come back.it is switching great cold and hot. Bruce Duncan

Where did you get the new sensors from? A bad new sensor does not surprise me, we have been seeing so many new bad parts, that in some cases the repair ends up being a used part. It looks like they are labeling the sensors 1, 2, 3, for Bank 1 and just Sensor 1 for Bank 2. Its an odd design for sure. But the Bank 1 and 2 Sensor 1s should both be swing their voltage from 0.1v to 0.9v in 1Hz. So one full up and down(cycle) within a one second.
Within the live engine data on your scan tool, it will be limited on what sensors you can see due to the age of the truck, but oxygen sensor data is highly important so you should be able to see their signals graphed. Then again, it sounds like its fixed unless something else was happening before.