If you have ever had a P0138 code click here!
 

I just want to get all this under one thread because there is a ton out there!
If you have ever had your Cherokee put out the P0138 code please reply to this thread all the symptoms and what it took to fix it.

It is kind of like a pole! That way we can pinpoint the most likely reason for this code.

Please only post if you have fixed this problem or know someone who has. (no opinions! )

Also please note if what you did made any MPG difference!

Thanks in advance to all who post!! I think every one will be able to get a lot out of this thread if we get some answers.

This code is for the after cat o2 sensor. Mines been acting up. Haven't replaced it yet. (waiting for pb blaster to soak)

Could be harness or after cat sensor.

I'll let you know.

I had a problem with mine a while back. The cause was due to the previous owner replaced the O2 sensor and spliced the new leads from the sensor to the old wires in the harnes with crimp splices and did not seal them in any way. These crimp splices eventually developed corrosion in the splices. This cause enough voltage drop across the splices that the voltage to the PCM dropped enough to cause the code. When I was troubleshooting this I could still watch a voltage swing from the rear sensor indicating that the sensor was working. But the swing was just to small because of the low voltage signal available at the PCM. I cut the splices out and soldered the wires back together and sealed the splices with self-sealing heat sink and everything was back to normal.

If you're wiring looks good. Pull the O2 sensor out (let it soak with PB Blaster a while and maybe even heat it up before trying to screw it out), then bench test it. If you don't know how to bench test it just google "bench test O2 sensor" and you'll find several instructions and even videos that will show you how.

I believe mine was original. With 168k on it. I replaced it after wiring checked out.

Took pb blaster, and bfh on a 1/2" ratchet.

I had the code, replaced the sensor. No problems yet. Just be careful because the wires pass pretty close to the driveshaft!

Has anyone bench tested an o2 sensor before and it checked out good, but when you replaced it it fixed the problem?

In other words is a bench test reliable for the performance of an O2 sensor?
It seems like all the bench test is telling you is if it has shorted out or not.

I have replaced the O2 sensor after the cat after getting that code. Had 102,000 mi on it. The check engine light is still off after about 600 miles!

I thought it would fix my sporadic rough idle though. It did not. Guess I better check the timing

1999 Cherokee, 4.0, AW4

CEL lit up a week ago. ScanGauge showed code P0138. I disconnected the negative battery cable and reattached it. CEL was off. After driving for about 15 minutes and 10 miles the CEL came on again. ScanGauge again showed P0138.

I disconnected the negative battery cable and replaced the downstream oxygen sensor the next day. I inspected the oxygen sensor wiring and fuses in the PDC. All seemed OK. I reattached the negative battery cable. I stared the Cherokee and the CEL was off and has remained off for the past week, about 200 miles of driving, 6 trips.

For my Cherokee, simply replacing the downstream OXYGEN SENSOR solved the problem.

x2 sandstone.

Usually when you go to replace an o2 sensor it helps to disconnect the negative cable to the battery. It's not really "necessary" because there is not a whole lot of current running into the plug, but due to needing to reset the code from the CPU it's needed. Usually 15 minutes of keeping it unplugged helps.

Hope this helps!

V/r,
-Gray.

Update
 

Mine ran for a couple thousand miles after the o2 sensor replacement with no light. Now I have it again. After checking the wiring (still good) I have come to the conclusion that a leak in the exhaust manifold is causing a bad O2 reading and maybe killing my sensors. Does this sound logical? I also found that the PO smashed the pipe pretty good right after the manifold. Exhaust restriction!
I have been wanting to fix the leak and pipe anyway. I'll give that header everyone has been bragging about a try.

P0138 and 0135...
 

I have been getting both P0135 and P0138 codes for the last few months and have done quite a bit of looking in this forum, wranglers and grand cherokees...lots of the same stuff comes up, so here is where I am presently:

113K on her (99')
- replaced both O2 sensors (stick with NTK)
- going to check wiring from sensor back and look for typical un-restrained wiring harness melted on the manifold or drive train.
- then checking fuses (apparently they typically go out when you get the 1038 code because of the short) - 15amp fuse in slot 24 is the one to check out.
- of course, be sure to check the current and voltage.
- more than likely not a PCM thing (of course we all hope so).

Smashed exhaust...
 

FYI J20jeepster -

That kink in the Exhaust does suck, but is a factory built design so that the front axle can travel. I remember the first time I saw that when I was installing a lift - I thought "what the hell", but came to find out that it is stock design. Getting a new header would let her breath soo much better - that is what I am eventually going to get around to - among other things.

I had a couple codes going: P0138 and one for the cat not working. Apparently my '96 Cherokee never had the Cat/ECM recall and the cat was really, really bad. I bought a couple Oxygen sensors and made an appointment at my local Jeep dealership to have the recall performed. They ordered a new cat, installed it, updated the ECM, installed the new Oxygen sensors I provided and I was off.

P0138 did not appear immediately but it did after 10 miles of driving. I went back to the dealership and since it wasn't directly related to the cat, I wasn't going to push for them to figure it out on their dime.

I did a lot of research: I checked the fuse (it was good), checked the ground (it was good), I even cleaned up all the battery terminals and all seemed well. I checked both the oxygen sensors and they were connected and in tight with no leaks. I really had no freaking idea. At least until I checked beyond the oxygen sensor connections. It appeared that just beyond the rear (downstream, just after the cat, before the muffler) oxygen sensor male connector (that comes from the vehicle), one of the wires was completely frayed from the connector - thus unable to send information to the ECM. I cut the connector off the oxygen sensor and the connector from the vehicle and spliced the wires together - taking time to make sure the wires matched up as the connector would have it - just take the time and write everything down.

VOILA! BINGO! THAT'S THE WHOLE ENCHILADA! That was my problem - a stupid frayed going into the male connector on the downstream oxygen sensor. I started the Cherokee up, cleared the code, and drove 40 miles for about an hour and a half. No code! I think I got it, Johnny!

Hi All...newby here.

Just bought a 1999 Cherokee in decent enough shape to take care of a few neglected items. The CEL came on the day I test drove it (it had been sitting awhile) and when I took it to my local guy for an inspection he read the code ...138. Told me it was the downstream O2 sensor, so I figured what the heck, I can change that, which I did... viola! New O2 sensor in, CLE out...feeling proud. Three days later it's back on. Read a lot of similar stories here, which why I decided to join the forum. Misery loves company. Now in addition to 138 I've got the large Evap leak fault 455, and he couldn't reset it with his OBDII reader. I decided to pull the negative cable off of the Battery to see if it would reset. In doing so, noticed lots of corrosion. Cleaned it up good, reconnected, and now the CEL is off! Drove it for 20 minutes, seems OK. Did I fix it?? Don't know but if the dang thing comes back on I'll repost.

Quick update:

Got a few days and over 100 miles on it now since cleaning up the battery terminals.... still OK.