specific oxygen sensor question

I've searched through dozens and dozens of pages on here and google, and have not been able to answer this question.

brief background: (1996 XJ 4.0)
Check engine light came on for no reason and within a day, it went off.
it did the same thing again, went on for a few days, and then off.

so i plugged it in and pulled code P0138
=O2 sensor, bank 1 sensor 2
=downstream of the cat O2 sensor bad

after doing more research it seems that perhaps it could be a short in the O2 circuit, opposed to simply being "a bad sensor"

I replaced my entire exhaust system with a borla header, carsound cat, and borla cat-back, about 2 years and 16k miles ago.
I also replaced BOTH O2 sensors with brand new Bosch units from Napa at this time (I was at 74,000 at the time=seemed perfect time to do it).

I physically checked under my jeep to look at the rear O2 sensor and wiring=LOOKS OK (not rubbbing drive shaft, and looks like a good connection)

QUESTION:
my understanding (after doing a lot of reading on here about it) is that the downstream cat on a 1996+ (OBD II) XJ, does NOT control any computer functions of air/fuel, but rather monitors the condition of the cat.
is this correct?
if so---
what would make a relatively new O2 sensor (only like 16k miles on it) go bad?
could it simply be that the sensor "went bad" and I should just replace it and forget about it?
or
would it signify that the (relatively new) cat is going bad?
or
???

please give me some ideas!!
I'm going to unplug the sensor this weekend and check the resistance between the white wires to see if it is within spec (5-7 ohms).
other than that i have no choice but to replace it i suppose

please help!!

CaptTrev

Capt, the P0138 indicates a high voltage on the sensor signal. This usually means a shorted sensor, either by carbon buildup or a possible short between the Vref and signal wire. I'd double check the wiring as it travels over the cat; if it's OK, then I'd just replace the sensor. I can't fault any of your logic.....

I agree, check to see if the wires from the sensor might be touching the exhaust pipe somewhere. Sometimes if you don't route them the same way they were, they might have some slack and end up touching the exhaust pipe..melting the insulation.. and shorting to the pipe.


MAP

I'll check over the wiring more thouroughly when I crawl under there after work tonight.

if the wiring is all good, then I'm going to pull the sensor and replace it.

if it went bad due to carbon build up on the tip---what would cause that?

i have a recent, "full" tune-up on there. (plugs, wires, cap n rotor, clean air filter etc)
and i use chevron with techron at every oil change (mobil 1, 10w-30 always)

why would more carbon build up on the downstream sensor, rather than the upstream?

should i :GASP: be thinking about replacing BOTH sensors tonight??

CaptTrev

I'd only replace the rear one with the code (If it ain't broke, don't fix it).

Cats get very hot, 1200 degrees or so, and that makes a harsh environment for the downstream sensor. Even the upstream header pipe dosen't get that hot. I was just guessing about the carbon; you can tell more about it when you pull out the old sensor. O2's usually last about 60-80K miles or more, so 16K is indeed a short life for it.....

I thought oxygen sensors lasted about that same amount of time as well (~60,000, at least that's what Bosch says) that's why I've been a little surprised with mine.

I have 113,000 miles on my 4.0L 99XJ. Bought it at about 23,000miles.
It just recently @ ~111,000 miles passed the smog check here in CA and it was pretty clean. I was thinking I might have to replace the oxygen sensors before I got it smogged, but since it passed so easily, I'm leaving the same ones.

MAP

i replaced the rear O2 sensor with a brand new Bosch unit from Napa-- $89.00
i plugged into my OBDII scanner, and cleared the code.

i had a 30 mile drive tonight, and so far so good. (no check engine light)

i bench tested the old O2 sensor, and found it to be within spec (5.6 ohms of resistance between the the 2 white wire contacts) :dunno:

it had a little carbon on the base of the sensor tip, but not enough to even classify it as carbon "buildup", in my opinion

something of note-- the new bosch unit looked a little different than the 2 year old (also bosch) unit. the new had a thicker external base, and fatter internal tip, and only 1 hole at the end of the tip, opposed to the 5 holes on the old one.
weird.

well, so far so good.

i'll keep you posted

CaptTrev

FWIW
I had a similar experience. I know you said you checked the wiring and maybe you have already checked this, but I had a pin in the plugs back out enough to not make a good connection. It wold work intermitantly and the shop I took it to tested the sensor and the connections and found them to check okay and suggested to replace the sensor. Still had the same issue. When I started to remove the new one to test it, I found the loose pin. Again, if you have not already done so, give a good tug on all the connectors and reasure yourself that the are all seated properly.