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Help with a PO140 code on a 1996 S10

First, have not been on this site in quite a while. Why ? Got sick and tired of working on cars. Well, now I got time on my hands and bought a 1996 S10 with 200,000 miles to keep me busy. This vehicle sure has not disappointed me, as it has kept me busy. Not going to boar you with all the details, but one item sticks out. Brought it to a local garage for state inspection. They told me I would be crazy to put $5 in this truck. They said the engine is shot. They said it sounds like it needs crankshaft bearings. I was pretty good at this years ago and just did not buy this. Turned out to be a water pump that was shot. Engine sounds really good now.
OK, my question…
Got a PO140 code. Indicates rear oxygen sensor, among other things. So far changed after Cat oxygen sensor. Erased code. Day later light back on !!
Thought vehicle set for 1 1/2 years not being used. Cat might not be working correctly. Put a container of Cat clean in fuel. Reset light, back on in a few days.
First question… What do you think ???
My scanner is bottom end. I’m thinking of buying a better scanner so I can read live data. Do you agree? If so, any recommendations ? I’m thinking about one at Harbor Freight. It is a Zurich ZR* for $82.99. Any thoughts? I could also use this one to help me with the ABS light being on (another post, another time).
Any help would be appreciated.

First answer. Clearly your cat is bad. For a P0140 , the downstream O2 sensor is the messenger not the cause. Changing it did nothing. Cat clean did nothing (no surprise there). So the truck needs a new cat.

I use OBD apps on my phone or android tablet. There are products that can communicate with the ABS systems. Maybe look to ObdFusion

New aftermarket cat for that truck is around $125. Not hard to change.

Thank You for the replies. It looks like I should just change the converter. I just didn’t want to just keep throwing parts at it until I get it right. I do think a new scanner is in my future. As I said, I haven’t worked on cars (much) for a while. I am getting back into it and kind of enjoying it. A better scanner that does live data and ABS would be beneficial. I do have an ABS light that is on. That is for another day.