So I'm looking to get a new cat and I want to get the shortest cat I can find and mount my muffler right behind it. So basically I won't have a bung for the o2 sensor. And from my understanding an o2 sensor works by detecting certain gasses. So would I just be able to zip tie the sensor somewhere out of the way? Or will I still get a cel? It seams too simple to work.

You'll definitely get a CEL hate to tell ya. You'll need some sort of bung for it

Any idea how much a shop would charge to weld in a bung?

Leave the downstream sensor plugged into the wire harness and secured properly. See what happens (or not).

The downstream O2 sensor is there to monitor catalytic converter efficiency. You can buy the bungs from Summit Racing or Jegs, and a muffler shop shouldn't charge more than 20 bucks to weld it in.

PO deleted the cat and engine light is always on, downstream sensor is plugged in but I want to make the engine light disappear..

what exactly is a bung...??

Negative.

Leaving the DOWNSTREAM o2 sensor out will not trigger a CEL.

Past owner is breaking federal law. What code is it throwing? (if you don't know, take it to an auto parts store near you, they'll be able to scan it for your). A bung is what the o2 sensor screws into.

Oh... Haha I just assumed. Whoops

97, 98 Cherokees do not turn Check light on if rear sensor has a problem, I think.
At least not if cat converter is bad. I knew some guy, who had piece of pipe installed instead of cat converter, rear sensor was still there as it should. No engine codes, no light on.
It is easy to check:
disconnect rear o2 sensor and start engine.
If you let sensor hanging, keep in mind that it is heated.
Well, you can install it inside driver seat to get it warm in winter, but 600F is a little warmer than you want.

The o2 sensor gets hot? I thought it relied on the exhaust to get its heat...

If it is 1 or 2 wire sensor, 3 and 4 wire sensors have heater.

Ah, I'm Renix. 2 wire iirc, prolly the source of my confusion.

You need the front O2 for fuel changes the computer uses it. The rear o2 can be removed, you just need to look up how to make a o2 Sim. It will tell the computer everything is good.

Re-read post #4.

Not sure about you newer guys, but my o2 sensor was snapped in half and was dangling down by my front diff for several months...and I only have one.

So the ECU must not care TOO much.