Truck is a 2001 Ram 1500 5.2l 2wd
It’s not for the o2 sensor unless it’s for a o2 sensor that is missing, I only have one o2 sensor and it is right before where the catalytic converter would be if I had one. (On the drivers side in the pipe at the top right of the first photo)

 

Sure looks like an 02 plug. You woulda had pre-cat and post-cat sensors to monitor catalyst efficiency.

 

I would agree, it should go up and over the trans. I'd imagine you have a CEL on? Depending if its front or rear, it could be running poorly

 

It’s not necessarily running poorly but does seem to be running rich and seems to have bad mileage and yeah I don’t remember the exact code I’ll have to double check but I know it was for the o2 sensor(s) The plug is indeed for a o2 sensor I unplugged that end with the dangling wires and I unplugged my o2 sensor that is present and checked it’s the same kinda plug. I don’t think my pre cat o2 sensor is working though I unplugged it and the truck runs/drives the same with it unplugged I also tried connecting it to the plug that had the dangling wires too and it didn’t run any different. It’s a new o2 sensor and I have one of those non fouler/spacers installed.

 

You don't need or want a non-fouler on the pre-cat. This one determines stoich -- which is kinda the whole point of EFI -- and so should always be fully in the exhaust stream

You should at the very least determine which plug is for pre-cat and get that working properly. Clear codes and see what happens

 

I actually had to use the non fouler in order to fit the new o2 sensor, the probe or whatever you want to call on the end of the sensor would hit the bottom of the pipe before the threads would make contact. The old o2 sensor that I removed looked to have been cut short to make it fit. The truck doesn’t have the stock exhaust someone fabricated and put this exhaust on the truck before I owned it.

 

Precats you need for fuel management. Postcat is to make sure the converter is working properly and has very little to do with fuel management. It will set a P0420 code and the engine light will come on. But yea that's an o2 plug.

 

After taking a closer look I realized the exhaust that’s on there was slipped inside of the pipe where the o2 sensor goes in and that’s what was blocking the o2 sensor from going down into the pipe, I cut it out and put in a new section of pipe there and have got the precat sensor in there without the non fouler now. Cleared the codes and the engine light came back but I’m hoping it’s just for the missing postcat sensor I’m borrowing and friends scanner and going to check soon

 

P0138 is the code that I am getting, that is for the post cat o2 sensor right? Can anyone confirm?

 

Yep, bank one sensor 2. That's the post cat sensor.