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post #1 of 22 Old 07-02-2020, 07:53 AM Thread Starter
Z_MAN
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P0138 and P0141

Hey guys, I've searched these forums and the cherokee forums and feel like I've done as much as I can before asking for help.

I got my 2000 xj the day before my State's stay at home order.... And mvd closed. Great. I need to register it but need emissions done. I have the 2 codes, P0138 and P0141. I'm relatively good at working on vehicles but suck at electronics.

I replaced my rear o2 sensor with a 12 dollar ebay one (which I now know was a bad idea). This was never a problem in my other vehicles, but I understand they're all different. The wires were torn from the previous owner off-roading. I thought this was the cause of my codes. I then read forum posts saying that we need ntk sensors only and felt dumb.

Because I thought I had fixed the rear, I ordered a new ntk for the pre cat sensor and installed it after that.

The codes both came back. I believe they refer to the post cat o2 sensor, towards the rear of the Jeep. I tested the sensor for continuity across the white wires and it is fine. The resistance was slightly over 7. I think 4 to 7 is acceptable. I've inspected all wires and they look fine. I checked my fuses, and none are blown. The previous owner said it had an aftermarket exhaust but I don't know what the stock ones look like.

What do you suggest ? An ntk for the rear as well? Test voltage at the harness to see if power is given to the sensor when the ignition is turned over? I need to register this thing and am frustrated. CELs drive me crazy. Thank you.

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post #2 of 22 Old 07-02-2020, 08:05 AM
CJ7-Tim
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Always use the OBD trouble codes to guide your diagnostics. The codes both mention wiring issues, you need to visually inspect and to use your volts/ohms multi meter to test wire continuity to the PCM, continuity to ground, input voltage, and O2 sensor performance. ALWAYS use NTK O2 sensors.


See this thread for some general O2 troubleshooting >> https://www.jeepforum.com/forum/f11/...e-bog-4402535/



P0138 JEEP - O2 Sensor 1/2 Circuit High

P0138 JEEP Possible Causes

Faulty Rear Heated Oxygen Sensor Bank 1
Rear Heated Oxygen Sensor Bank 1 harness is open or shorted
Rear Heated Oxygen Sensor Bank 1 circuit poor electrical connection

P0141 JEEP - O2 Sensor 1/2 Heater Performance

P0141 JEEP Possible Causes

Faulty Heated Oxygen Sensor (H2OS) Bank 1 Sensor 2
Heated Oxygen Sensor (H2OS) Bank 1 Sensor 2 circuit fuse
Heated Oxygen Sensor (H2OS) Bank 1 Sensor 2 circuit open or shorted to ground
Heated Oxygen Sensor (H2OS) Bank 1 Sensor 2 circuit poor electrical connection

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post #3 of 22 Old 07-02-2020, 08:22 AM Thread Starter
Z_MAN
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Thank you and I'm doing my best so far. I have generally had luck with other vehicles only needing to do mechanical work.

I watched a youtube video about testing the harness. I did that on my aftermarket sensor, there is continuity on the white wires, and the resistance on the ohm meter is 7. The wiring harnesses look good front and rear as far back as I can see. No bare wire, no breaks, etc.

To test the power to the O2 sensor, you measure the white/green wire on the harness going to the PCM right? should be the standard 12v?

I don't want to throw parts/money at this and want to diagnose correctly, but do you think I SHOULD get another rear O2 sensor? Maybe the 12 dollar china one is causing the issues?
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post #4 of 22 Old 07-02-2020, 08:42 AM
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I had those exact codes. I replaced the O2 sensor which got ripped off, and fixed a short that i had because a piece of the wire was stripped bare when the exhaust fell and ripped the O2 sensor out. It left me stranded twice before i was able to find it and fix it.

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post #5 of 22 Old 07-02-2020, 09:41 AM
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Quote:
Originally Posted by Z_MAN View Post
Thank you and I'm doing my best so far. I have generally had luck with other vehicles only needing to do mechanical work.

I watched a youtube video about testing the harness. I did that on my aftermarket sensor, there is continuity on the white wires, and the resistance on the ohm meter is 7. The wiring harnesses look good front and rear as far back as I can see. No bare wire, no breaks, etc.

To test the power to the O2 sensor, you measure the white/green wire on the harness going to the PCM right? should be the standard 12v?

I don't want to throw parts/money at this and want to diagnose correctly, but do you think I SHOULD get another rear O2 sensor? Maybe the 12 dollar china one is causing the issues?
To check the heater circuit, backprobe the pins I've highlighted. Make sure there's no continuity to ground, and you should get alternator voltage. (I'm guessing Cali emissions, but It looks the same as federal). If you don't have a backprobe kit, sewing needles work just as well.

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post #6 of 22 Old 07-02-2020, 09:48 AM Thread Starter
Z_MAN
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Quote:
Originally Posted by Pinoy59 View Post
I had those exact codes. I replaced the O2 sensor which got ripped off, and fixed a short that i had because a piece of the wire was stripped bare when the exhaust fell and ripped the O2 sensor out. It left me stranded twice before i was able to find it and fix it.

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interesting. Did you use the NTK sensor? I didn't and am wondering if that's an issue.

bare wire? was it easy to spot? My harness looks amazing still for the age. how do I test for a short with my meter?
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post #7 of 22 Old 07-02-2020, 09:59 AM
azhang253
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interesting. Did you use the NTK sensor? I didn't and am wondering if that's an issue.

bare wire? was it easy to spot? My harness looks amazing still for the age. how do I test for a short with my meter?
I've been told that NTK's are essentially required, but I've never used anything else. There are many threads talking about problems with non-ntk O2 sensors.

I found this video on how to probe sensors:
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post #8 of 22 Old 07-02-2020, 10:19 AM
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Quote:
Originally Posted by Z_MAN View Post
interesting. Did you use the NTK sensor? I didn't and am wondering if that's an issue.



bare wire? was it easy to spot? My harness looks amazing still for the age. how do I test for a short with my meter?
The brand is kwiksen. It works great, no issues. Even though it was so close to the connector, everyone missed it. It was an easy fix. Butt spliced the wire, taped it and put in a new loom. I knew there was a short somewhere because it was draining my battery. It was blowing a fuse every time i turned it over and i went through about 30 fuses before i figured it out.

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post #9 of 22 Old 07-02-2020, 10:52 AM Thread Starter
Z_MAN
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Quote:
Originally Posted by Pinoy59 View Post
The brand is kwiksen. It works great, no issues. Even though it was so close to the connector, everyone missed it. It was an easy fix. Butt spliced the wire, taped it and put in a new loom. I knew there was a short somewhere because it was draining my battery. It was blowing a fuse every time i turned it over and i went through about 30 fuses before i figured it out.

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Interesting...again! I have had my battery die twice if not driven for a few days. I really cannot see any bad wiring at all. Granted I didn't pull back the sheathing holding the wires together. Only as far back as I can see. I really hope I can fix this.
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post #10 of 22 Old 07-02-2020, 11:05 AM
Pinoy59
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Interesting...again! I have had my battery die twice if not driven for a few days. I really cannot see any bad wiring at all. Granted I didn't pull back the sheathing holding the wires together. Only as far back as I can see. I really hope I can fix this.
Yeah thats what was happening to me but not all the time. I would pull the loom off and really inspect the harness up to the transmission. Its worth a look.

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post #11 of 22 Old 07-05-2020, 05:10 PM Thread Starter
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Hey guys I looked at the wiring as high as I could, up to the top of the transmission.... Looks good. Great actually. Do we have a relay for our sensors? My fuse is fine.

I didn't back probe the harness because I'm confused. The diagram shows 4 sensors, 2 upstream and 2 downstream... I only have 2 sensors.
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post #12 of 22 Old 07-05-2020, 05:36 PM Thread Starter
Z_MAN
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I have a decent code reader that does live data. Does this mean anything? The one reading 1.2v was not moving much. Only 1v or higher. The top one, bounced up and down a lot.
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post #13 of 22 Old 07-05-2020, 05:47 PM Thread Starter
Z_MAN
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I found this advice in another thread.

"If you see a voltage higher than 1-1.2 volts then unplug the sensor and see if the voltage goes away, if not then the wiring is shorted to power somewhere, if the excessive voltage goes away after unplugging the sensor then the sensor is faulty."

I unplugged it and the voltage stayed at 1

Again, I do have a 12 dollar unbranded o2 sensor on back. Does my test of unplugging the sensor and still having 1v mean the cheap sensor is ok?
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post #14 of 22 Old 07-05-2020, 06:03 PM
CJ7-Tim
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Numerous threads on different 4x4 Forums tell of drivability issues when installing Bosch or generic auto part store brand O2 sensors on the Jeep 4.0L.

ALWAYS install NTK O2 sensors.


1991-2001 HO O2 diagnostics:

The O2 Sensor is also the main sensing element for the Catalyst and Fuel Monitors. The O2 Sensor can fail in any or all of the following manners:
· slow response rate
· reduced output voltage
· dynamic shift
· shorted or open circuits



Response rate is the time required for the sensor to switch from lean to rich once it is exposed to a richer than optimum A/F mixture or vice versa. As the sensor starts malfunctioning, it could take longer to detect the changes in the oxygen content of the exhaust gas.

The output voltage of the O2S ranges from 0 to 1 volt. A good sensor can easily generate any output voltage in this range as it is exposed to different concentrations of oxygen. To detect a shift in the A/F mixture (lean or rich), the output voltage has to change beyond a threshold value. A malfunctioning sensor could have difficulty changing beyond the threshold value.

For the HO years, 91-02, the O2 sensor has 4 wires. One wire is the 12-14 volt power that comes from the O2 sensor heater relay on the passenger side firewall (unless they moved them in later years to the relay & fuse boxes?), and that powers the internal heater in the sensor so that the sensor can work at idle, and almost immediately after start up. Loss of that power will hurt gas mileage even with a good O2 sensor.

Two black wires are grounds for the heater power and O2 signal to the ECU. A poor ground will limit current to the O2 sensors heater or cause an error in the O2 sensor output voltage read by the PCM (ECU) causing poor mileage even with a good O2 sensor. Test the 2 ground wires with power off. Test between the wire end at the sensor and the battery negative ground post. It should read less than 1 ohm.

The last wire, #4, is a signal feed wire, 0-1 volts, from the O2 sensor to the PCM (ECU) sensor. The O2 sensor is an O2 concentration sensitive variable voltage generator. At optimal O2 concentration the O2 sensor puts out 0.45 volts.

At idle that voltage should read 0.1-0.9 volts oscillating quickly back and forth roughly once every second. At 2000 rpm it should run between 0.4 and 0.5 volts max (in park). A digital meter can NOT be used for reading the O2 sensor voltage, but it can be used to test the grounds and the 12-14 volts to the heater. You must use an old style analog meter with the needle gauge on the display to see the voltage swing back and forth.

If the O2 sensor readings are not right, say they read .1 volts or .8 volts steady, you have a problem. BUT before you blame the O2 sensor make sure it has good wiring, and make sure the proper voltage is feeding it, by turning power on, engine off to read the engine off voltage feeds (12-14 on one, and ensure the ground wires (power off) reads less than 1 ohm to the battery negative post.

If any one of the grounds or the O2 sensor to ECU/PCM wires reads a high voltage chances are the wiring harness is damaged and the 12-14 volts for the O2 sensor heater is getting through a bad spot in the wires insulation and contacting a ground wire or the O2 sensor wire feed to the ECU/PCM. This is not the O2 sensors fault, but a wiring problem.

A leaky exhaust system or leaky fuel injector(s), or bad compression or leaky valves, bad plugs, wires, cap, rotor, HV coil, and so on, or combination of these, can also cause a lean or rich condition that gives you high or low O2 sensor readings that are not the O2 sensors fault, so try and verify those other items also before buying parts like an O2 sensor to fix your problem.

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post #15 of 22 Old 07-06-2020, 12:08 AM Thread Starter
Z_MAN
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I do not have any driveability issues currently. Only the CEL and codes p0138 and P0141. I want to register this xj and do not like going to mechanics. Does the live data option, 1.2v downstream reading, And constant 1.2v plugged or unplugged reading help is at all? To pinpoint anything?
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