Rear O2 sensor

97 XJ 5 speed check engine light ,code 138, high voltage ,bank 1, sensor 2. No big deal to replace it..but...
The engine runs fine,no change in fuel mileage. The question is...does this rear O2 sensor have any effect on engine performance?
Oh...I did replace the convertor about 2000 miles ago because it fell apart.The O2 sensor was just swinging in the breeze and did not trigger a CEL.I reused the same O2 sensor. No CEL until a few days ago. The replacement convertor now has an internal rattle when warm.. Thanks

I don't believe the rear sensor has any effect on mileage or performance, but is used as a sort of double-check of the front one which does. I have noticed on my 99 and on a subsequent Liberty that the sensor, when it gets a bit older, is sensitive to fuel, and would often come on after a hot start. Often a refill with a different brand of fuel would clear it for months. I just got used to ignoring it until it went off again. If you have a code reader that turns it off, you'll often find it stays off. The XJ system requires something like 40 starts before a code clears itself.

Your front O2 sensor oscilates above and below 450 mv. This is to switch your fuel/air ratio rich and then lean. To oversimplify your vehicle creates 5 gasses that the cat is trying to burn up within it. Some burn better rich or with a low oxygen content and some burn better lean or high oxygen content. The rear O2 serves as a check on the cat. Remember it only sees oxygen so it should remain fairly constant because the vast majority should be burnt up. It shouldn't however have an effect on mileage or performance.

Yes, I reset the light with the scanner.....
Ok, so before when the cat rotted out ,the O2 sensor was hanging from the wire with the rusted bung still attached...Drove around for a few hundred miles and no CEL... It should have triggered a code...
About 10 years ago I got a recall on my 96 XJ, something about a computer reprogram so if the cat fails the CEL comes on. That must have been related to the rear O2 sensor?

So....I replaced the O2 sensor and after about 10 miles the CEL came back on...same code....I looked very carefully at the wiring harness to the sensor and saw the area where the drive shaft had worn through the plastic cover and a wire....The harness was secured so I didn't check before replacing the sensor,wire broke from pulling on the connector.. 20 bucks no big deal.