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Check Engine light

Walt from Ky

NAXJA Forum User
Tucson, Arizona
99 XJ Sport, 4.0, Aw4, NP 242, 32,340 on ODO. My check engine light came on driving from work last night. Engine and trans operate the same, smooth and normal. All fluids and pressures are normal. Alt ok. No odd noises and nothing looks out of ordinary under the hood. Couldn't find anything in the FSM on the check engine light or if it comes on for scheduled maint. I was wondering if there was a way I could read the codes before I go to the dealer and pay them? Also, does the light come on for scheduled maintenence? TIA
I just read the thread in mod tech about Bosh Plats causing a misfire code on late model 4.0's. I have Plats that have been about 6 months, with no problems up to now. I have already bought a set of OEM plugs to replace them and I'll do that before I have the codes read.
see if you can go to the autozone in your area and they might still read codes (they don't do that in CA anymore :( ). In any case, I think if it was a misfire you'd hear it or feel it as you're in the XJ.... I have bosch 4+ in mine and they rund fine.....
Also look at it this way: you have been running those for 6 months with no complaints, so it might be something different that;s causing the trouble

Thanks Kejtar for reminding me about Autozone. Went by on my way ome and got the following reading. PO138, high voltage on O2 sensor #2. I looked in the FSM and they do not specify 1 or 2, just front and back. I think it's the back cat sensor thats bad but I'll have to check both before I buy one. One question, should I go ahead and replace both sensors while I"m at it? I'm a little surprised that the sensor has gone bad with so few miles on it. TIA.
Actually Walt,
Contact your dealer about emissions compliance -- the system should run for 50K before maintenace and/or failure (fed req) so they could prolly help you out on the o2 sensor under the emissions warranty (it's a vehicle-based warranty for all federally certified emissions packages)...

However -- the after-cat sensor is prone to road and silt damage (duh, on a Jeep what were they thinking?) so it's likely that that damage could be the cause of this failure --

After you've checked the harness and brushed the area around the rear o2 sensor, return to Autozone and have them press the reset button on their tester twice (to clear the stored code) -- see if it returns -- that o2 sensor is something that they carry and offer a pretty good price on if it comes to that ...
Thanks satan. I'll try to get it working first and if that doesn't work I'll try to get the dealer to replace it. Last resort I'll buy one at Autozone. I assume it won't hurt anything to drive a couple of days with the light on. The engines running fine.
My code book says "Pre-catalyst 02S Shorted to voltage" this would be the one in the down pipe
In addition.....

That post O2 sensor looks to be as well placed and accommodated on these XJ's as most every vehicle i have found, and better than others- so don't think that is much of a deal. If the light stays on then is a hard code and is a sign of a big problem, if goes off not necessarily.

Have posted in this Forum several times that i have personally experienced sometimes major problems with Bosch Platinum plugs in a variety of vehicles. Does not surprise me that Chry would post a bulletin of this misfire deal while using them on these. Something about them is not right.

If you find you must replace the O2 sensor with your own money, not sure i would buy a Bosch from AZ. So far i have found is a "you get what you pay for" syndrome with these and have found this brand seriously lacking...much as with their plugs. Usually OEM is the best route to go with ALL sensors and this one is no exception.

O2 sensors are testable for most parameters. Is not too hard to test them using a DVOM and instructions. Just to replace it, esp if you are paying, most times is not necessary.
To cold and rainy to look at the O2's last night, no garage. Started to go to work this am and the light went out. Hmmmm, makes me think that I have connection problems. We did have a long and salty winter here in KY. Should be warm and sunny today so I'll crawl under her with some dielectric and cleaner and do the deed and also check voltage, etc. Judd, I agree with you about the plats and they will bite the dust this weekend. Also I'm learning from reading this board and from personal experience that you usually can't beat OEM products. You do get what you pay for.
my 02 sensor is reading high voltage. this can be caused by the harness being damaged and it is. could the running too Lean code be linked to the O2 sensor harness being messed up, both say bank 1. and if not any ideas on why im getting a lean code?
my 02 sensor is reading high voltage. this can be caused by the harness being damaged and it is. could the running too Lean code be linked to the O2 sensor harness being messed up, both say bank 1. and if not any ideas on why im getting a lean code?

Holy Necro-posting! Dug this one up from Tut's tomb?

For this you should have just started a new thread.

Please, ALWAYS give the basics--year, engine, trans, etc.