XJ OBDII codes

SpydyrWyr

Member
Joined
Sep 26, 2005
Location
Raleigh
Figured I'd post this in newbie tech since i'm sure the answer is obvious to jeepers. Anyway got a '99 XJ Classic, 4.0L/AW4, 118,000 miles. Running fine, but the check engine light came on. It's driving my crazy since we just bought the damn thing Monday. I won't have access to a diagnostic tool untill this weekend, so I was wondering if there is a way to jump out any of the pins on the harness to get the check engine light to flash the codes??? I've done this several times on Yotas, but never had any Jeep experience.
 
ok, so i tried the key trick, holding down the trip reset button then turning the key on, then releasing. It went through the normal 111111 through 999999, then it's supposed to display the vehicle serial number then codes, but nothing happened after the 999999. Any suggestions?
 
Never heard that one.


If you live near wake forest, drop by the house and we'll hook the determinator up to it. As far as I know all OBD2 stuff only communicates via a scan tool. You didn't fill it up while it was running did you? :lol:
 
nope. Thanks for the offer, but I guess I can wait untill this weekend to use my buddy's LAUNCH diagnostic tool. It's just irritating me and I want to hurry up and figure out what's throwing the dash light. We bought it Monday morning for my wife, and monday on her way home from work, she called me and said, "guess what...."

I've been googling and browsing around pirate and tried the two different methods people suggest.

This seems to be a pretty thorough list:
http://jeephorizons.com/phpbb/viewtopic.php?t=2719
 
That trick only works on 97 and older, not newer, you will need a scanner to read it. Auto Zone will pull the codes for free. And so will Advance. If you were close to Sanford, I could do it.
 
Cool... I guess I'll quit workin' the key back and forth then. Some guys on pirate said they pulled codes that way on an '01, a '00 and so on. Maybe they weren't XJ's. Either way, I didn't have any luck. The holding the odometer thing definately does something, makes the gauges and indicator lights go through some weird cycles, just like described in that walkthru, but no codes.

Thanks to all you guys for all the help and offers to pull the codes for me. I'll take care of it tomorrow or Friday. It's runnin' like a champ, I'm really hoping it's some BS code.
 
Thanks Lee! I'm planning on going over to Steve-o's tomorrow to scan it and possibly clear it. I'm nervous about what kinda code is going to pop up, the last thing i want to do is have this thing already start to nickel and dime me to death!
 
I wouldn't doubt that at all. But let's say it's throwing an O2 sensor code. Then the question remains, is the sensor the problem, or a symptom? I guess I'll just have to burn that bridge when I get to it.

Thanks again for all the help.
 
Usually the problem, they don't last forever. And you have two, one near the manifold, one near the CC. Usually it's the manifold, but sometimes both. I replaced one on the wifes XJ, only to have the check engine come back on a week later, had to replace the other, scanner should tell you which.
 
Don't just pull the code and kneee jerk replace what ever sensor sent the code, there may be another reason the sensor coded.

O2 sensors can trip if the engine temp is too cool, as they don't get hot enough to correctly register AF mix, can set Cat codes as well, might even set a too cool code ( usually newer vehicles)

Read the freeze frame data associated with the code as well, engine coolant temp (ECT), AFM reading and such. Also check the air filter, a rich mix has set many O2 codes due to a plugged up filter. as well as AFM.

Just a few ideas, you gotta decode it based on what you find.

Kevin
 
Yeah, I had the same train of thought, that's why I'm nervous to find out what it is and then even afterwards, whether its a root cause or just a symptom of another problem. Hopefully I can draw some conclusive information tonight. More than likely I'll author a "99 XJ throwing code XXX, help!" thread in the next couple of days. :D

Thanks y'all,
-Dan
 
Rob said:
Bet ya it's the O2 sensor.
Click to expand...

We have a winner!!!

Well at 118,000 miles, it's about that time. The code is P0138 - O2 Sensor Circuit High Voltage (Bank1 Sensor 2). Am I correct in saying that's the downstream (post CAT) sensor? Of course it would be the expensive one! I've read that sometimes the 1st sensor can cause the second to throw a code, but I'll probably replace them both anyway.

If it turns out the sensors are OK and that something else was the root cause, I might be screaming for help since i'm a Jeep newb.

Thanks everybody,
-Dan
 
Dan, you need to take the scanner and see if the voltage on the 02 sensor is fluctuating at all, you can pop an after cat 02 sensor code with a bad cat also (although usually an eneffiecent catalyst effeciancy code). The scan tool Steve has should be able to do a datastream so you can observe the 02 sensor voltages, If not give me a shout and I can take a look at it.
 
Steve's LAUNCH tool is not liking the XJ. I can't pull any freeze frame data or datastream for the O2 sensor. Do you have a capable scanner at home or do I need to visit you at work?

Thanks Lee, I owe you a :beer:
 
Yep, what Lee said,

while yer under there, try popping the cat with yer hand or something solid but doesn't clank, listen for a rattle, bad cat will often rattle but you may not hear it with all the other road/engine noise.

while checking the data stream of the O2 sensors, watch the flip, from rich to lean, should be even, if it seems to hang rich, the cat may have been comprimise as well, and it may be an indication of a vacuum leak at the intake.

Kevin
 
Well here's hoping it's just a sensor. I can't believe I'm already getting nickeled and dimed by a vehicle I've owned for 3 DAYS! This is horse chit. The Yota Gods are pissed at me for buying a Jeep. :D
 
J ust
E mpty
E very
P ocket

And and you thought it was a joke ? !

Kevin
 
Upstrem o2 pattern should be a fairly quick switch from .25v to .85v or in that range. The key is the top and bottem of the range and switch time. The rear o2 should hold in the .45v range fairly tight if the cats ok. The idea is to make sure the cat is working right. The 4.0 has a history of cracking exhaust manifolds and~or busting studs off and leaking. That will toss o2 codes as well as the cracked beds in the cats themselves. This is on a fully warmed up engine. Check the vent setup off the back of the valvecover as well, those things plug and cause wierd fuel issues, some of which will throw codes. :beer:
 
Well I've got a little update that has me puzzled. So I haven't had a chance to do anything but pull the code and find out that it was the downstream O2 sensor. I've been travelling a lot for work and been slack, so i haven't had time to do anything else to it. So the wifey calls me last night and says, "guess what, the jeep likes me finally!" When i ask her what she means, she tells me that the Check Engine light is no longer on. So I have her turn the key to the "on" position to make sure the bulb isn't burned out. Well it comes on, then she starts it, then it goes off as if all is well. Anybody ever seen this kinda "self repair" I mean i've heard of misfire codes and stuff like that clearing themselves, but never on O2 sensor?
 
SpydyrWyr said:
Well I've got a little update that has me puzzled. So I haven't had a chance to do anything but pull the code and find out that it was the downstream O2 sensor. I've been travelling a lot for work and been slack, so i haven't had time to do anything else to it. So the wifey calls me last night and says, "guess what, the jeep likes me finally!" When i ask her what she means, she tells me that the Check Engine light is no longer on. So I have her turn the key to the "on" position to make sure the bulb isn't burned out. Well it comes on, then she starts it, then it goes off as if all is well. Anybody ever seen this kinda "self repair" I mean i've heard of misfire codes and stuff like that clearing themselves, but never on O2 sensor?
Click to expand...


Most OBD2 codes will "self clear" after 40 or so passes under the monitering maps in the ecu. Basically the ecu self tests things at prescribed times, some are continous moniters and other are tested as part of a drivecyce. Sound to me like it passed the rear o2 enough time to kill the light. Odds are the code is still stored. In order to ease diagnisos of anything in the furture, I'd read the stored codes, make a mental note of them and clear them. Misfire is a continous moniter and will self clear fairly quickly.
 
I'm just wondering if there actually is a problem or not. I've heard that sometimes drastic change in driving style (like from owner to owner) can sometimes throw an O2 sensor code. I'm not sure how long this thing sat when it was for sale, maybe a few weeks. I'm hoping there really isn't a problem, but that's just wishfull thinking. What do y'all think? Just wait and see if it throws another code?
 
SpydyrWyr said:
I'm just wondering if there actually is a problem or not. I've heard that sometimes drastic change in driving style (like from owner to owner) can sometimes throw an O2 sensor code. I'm not sure how long this thing sat when it was for sale, maybe a few weeks. I'm hoping there really isn't a problem, but that's just wishfull thinking. What do y'all think? Just wait and see if it throws another code?
Click to expand...

Read it, clear it, drive it.
 
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