96 P0138 and sensor circuit

I probably got seven thousand miles out of my last NTK O2 downstream, and I replaced both as a pair. Bit the bullet and purchased a Denso unit because I really didn't feel like ordering another one, but now I think I can return the NTK to the exhaust because replacing it made little difference.

I noted previously that my sensor voltage didn't fluctuate from one volt. Replacing the sensor, I have fluctuation again, only it climbs up to around 1.4v on occasion. Now, it just sits at 1v.

I repaired the grounds at the dipstick tube, but only after snapping the end of the stud off and cutting everything to free it. the black/red wire was just about gone at the terminal. Back to square one, no change.

Would cleaning the sensor make any difference? I'm letting the exhaust cool down before I get back into it, but I plan on cleaning the NTK and putting it back in.

The sensor receptacle on the harness looked good when I had it apart last time, all of the pins were protruding properly. Is anyone familiar with what tool releases these pins?

Could easily be a wiring harness problem, I just went thru this!

I'm suspecting the connections to the rear harness from the firewall at present, but I'm going to try cleaning first.

Sensor came out near spotless, which is surprising because the fuel economy has been terrible. I went ahead and pulled the whole lower harness after I noted a rubbed spot.

Also, I can't seem to get pictures to link.

There's a tab on the tail end of each receptacle which releases the retainer. From there, it looks pretty simple to remove whichever pin you want. I'm going to shrink tube a couple of wires with crumbling insulation, but I don't think that was the problem.

After having disassembled the harness, and scrubbing it in the sink, it would appear to have a bit of corrosion on the pins on the sensor end. I'd say it did pretty good for twenty years.

We'll see what happens once it's fixed, cleaned, and wrapped back up in new split loom.

Must have been the corrosion.

Took three drive cycles before it came off of one volt, and it went higher first.

For what it's worth, it's not hard to isolate and remove the lower harness to the sensor.

.04v was the lowest I saw. After seven drive cycles, it still fluctuates near to the upstream sensor's reading. No check engine light.

Good job. I've found that low voltage circuits like these are very suseptable to high resistance and poor connections. The high heat of the cat dosen't help either. Thanks for posting your results.

Looking back, it could have been the grounds at the dipstick, but I only started the engine a few times in the driveway after repairing them while watching the voltage on a scan tool. All the same, the harness looked pretty bad in places. Transmission fluid appeared to be the reason the insulation crumbled in places, much of it was almost melted together, and the connectors were so covered in oily dirt accumulation that at first I couldn't tell how they came apart. Half of the split loom was junk.

tl;dr change your output shaft seal even if it only leaks when parked on the back bumper.
Make three or more drive cycles before assuming a repair made no difference.

Overland said:

Sensor came out near spotless, which is surprising because the fuel economy has been terrible. I went ahead and pulled the whole lower harness after I noted a rubbed spot.

Also, I can't seem to get pictures to link.

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From everyrhing i have read the downstream o2 has little to do with fuel trim.