Just recently got this 99 sport and the check engine light was on for a low voltage issue to the rear O2 sensor (can't remember the exact code at the moment). I replaced the rear O2 sensor and checked the wiring and it looks good. Also checked the #17 fuse in the engine bay, I read that was the O2 sensor one to check. All looked good so I disconnected the battery for a few hours then hooked it back up and the check engine light was off. After about 24 hours it popped back on again. Do I need to have the code cleared by a mechanic with the tool or what else should I check at this point?

 

*You do not have to get the code cleared by mechanic
*Did you scan it again to make sure its back on for the same thing P0138?
*Sometimes you just have to "Drive it off". Ive never had this problem with the jeep, but when i had my Chrysler, if it had a CEL on, and i fixed the problem, disconnected the battery, it would still come back on, and i'd have to drive something like 25 miles to get it to go off.
*You should replace the upper sensor too, as that is probably at the end of its life as well.

Well, there's a few possibilities;

1. Did you check the fuse with a multimeter or just visually? it is possible that the fuse looks fine, and is not. Get a multimeter on that if you haven't already.

2. You may have gotten a faulty O2 sensor, which is completely possible. Did you get a cheap brand? Use only Mopar, or NTK. Even then, its possible to have received a faulty part.

3. There could be a short in the wiring, somewhere between the O2 sensor, and wherever it leads to. I do not know how to tell you to go about checking this, but i have heard of this happening.

4. If its after the cat, maybe your cat is dead, and its chewing up your O2 sensors. I doubt that though, since you said the code came right back.

 

Fuse F17 in the PDC is for the Headlamps and Data Link Connector.

Refer to the PDC image below to find the O2S Heaters fuse.

 

Here's a pic that shows the NTK Oxygen Sensor that you should be using.

Note: A new O2S downstream along with an old O2S upstream can cause prpblems. They should be changed together.