1998 Cherokee 4.0. threw a P0138 O2 SENSOR CODE. SCANNED IT AND THE VOLTAGE REPORTED ON MY equs SCANNER SHOWED exactly 1.00 VOLTS NO MATTER IF IT WAS PLUGGED IN OR NOT and hot or cold. THE 15 AMP FUSE UNDER THE HOOD IS GOOD AND HEATER POWER IS GETTING TO THE PLUG AT THE SENSOR CONNECTION. THE SENSOR IS NEW TWO MONTHS AGO BECAUSE IT WAS TERRIBLY SLOW TO REACT AND IT WAS DUE FOR EMISSION TESTING with 110,000 miles. I LOOKED AT THE WIRING HARNESS AND PLUG AND ALL SEEMS WELL. is IT NORMAL FOR THE SCANNER TO SEE/REPORT 1.00 VOLTS EVEN IF IT IS DISCONNECTED? the S1 sensor crosses over and under .5 volts as normal as soon as warmed. Runs great and no drivability issues. I did not get to actually check the O2 voltage at the plug yet. Any thoughts to check or common failures here

Thanks

 

I have been searching the vast forum archives on this subject all morning and looking at most of the suggestions but still cant understand why sensor one ( Manifold ) reports normal operation while the sensor two ( after cat ) is solidly at 1.00 no matter what I do . Even if disconnected, Is this some default number generated when the numbers are totally out of line.Need to drive it to work today and will look again tonight? Any gurus out there with some help would be greatly appreciated. I want it fixed but I want to understand it even more.
Thanks

 

I have been dinking around now for 3 months when ever time permits with connectors and wire harnesses and found nothing unusual. Finally ordered new NTK sensors ( front and back ) from Rock auto and swapped them out along with removing, thourly cleaning and retorquing the grounds at the engine block. The stupid thing works as it should for 3 weeks so far and no check engine light. Still runs great and May have gotten a tiny increase in mileage but never go the same place so hard to confirm.Thanks for the hints in the other threads related to P0138 search.

 

Wow, don't think I have seen a thread that starts with a question and follows through with the answer without a single member chiming in. Well done on your repair.

 

Gotta ask. What brand oxygen sensor did you "originally" install (before the NTK)....

I have found NTK to work best on 91-01 XJs. It's the OEM exact replacement sensor.

 

I do not remember the brand for sure but it was likely a Bosch sensor. I have had good luck with them in the past. It was from my local CarQuest store. I read so much on this forum regarding this issue and the suggestion of using OE brand. By searching P0138 here this is where I have ended up for the moment. Thanks for all the posts from folks in the same position. These are wonderful vehicles but can be a bit Quirky at times.