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Unread 01-22-2009, 07:39 AM   #1
bgrames
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4.0 cherokee, code P0138, remedies tried and problem still exists

2000 Cherokee 4.0 L, check engine light comes on. I had Autozone check the code. I got a reading of P0138 which is the oxygen sensor. I replaced the sensor but the check engine light stayed on. However, the check engine light does turn off when I fill the gas tank and does not turn on till the tank drops a little. As suggested, yesterday I bought a new gas cap and drove around for a while but the problem still exists. What could be the problem and what are the possible remedies.

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Unread 01-22-2009, 07:44 AM   #2
xjfever
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I would disconnect the the positive cable on the battery, then touch it to the neg. cable for ten seconds or more. Then replace the cable, and see if the light comes back on. If it does I would take it back to a-zone, and have them pull the codes again. You may have a different problem now.
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Unread 01-22-2009, 10:02 AM   #3
Paradise XJ
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Quote:
Originally Posted by xjfever View Post
I would disconnect the the positive cable on the battery, then touch it to the neg. cable for ten seconds or more. Then replace the cable, and see if the light comes back on. If it does I would take it back to a-zone, and have them pull the codes again. You may have a different problem now.
I agree with disconnecting the battery, but I'm not quite sure I'd short it on the other side of the battery itself. I should be enough to just disconnect the battery, and wait awhile, don't know if it's 10 seconds or 1/2 hour but when you hook it back up, go pos first, then neg, then let the ECU relearn, or reprogram the fact that you've changed out the O2.

Do you have more than one O2 (I have a RENIX, so I dunno about a 2000).

Just my.02 on the O2
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Unread 01-22-2009, 03:43 PM   #4
Ed Rico
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Quote:
Originally Posted by bgrames View Post
2000 Cherokee 4.0 L, check engine light comes on. I had Autozone check the code. I got a reading of P0138 which is the oxygen sensor. I replaced the sensor but the check engine light stayed on. However, the check engine light does turn off when I fill the gas tank and does not turn on till the tank drops a little. As suggested, yesterday I bought a new gas cap and drove around for a while but the problem still exists. What could be the problem and what are the possible remedies.
P0138 is the high-voltage code for the downstream O2 sensor. If the sensor does not generate a variable voltage output after a fixed amount of time, the PCM substitutes a fixed value of 1V and sets this code. More than likely, the new sensor isn't working properly because the internal resistance heater isn't functioning properly. Check the fuses in the underhood PDC (Both O2 sensor heaters are fed by a single 15A fuse) to make sure they're all working, you should also take a look at the ground ring connectors mounted to a stud on the passenger side block adjacent to the oil dipstick. Finally, take a good look at the wiring harness running along the trans tunnel to the downstream O2 sensor, it's common for the harness to become damaged from contact with the driveshaft.

-Ed Rico
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Unread 01-22-2009, 04:40 PM   #5
BrunoS
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Originally Posted by Ed Rico View Post
P0138 is the high-voltage code for the downstream O2 sensor. If the sensor does not generate a variable voltage output after a fixed amount of time, the PCM substitutes a fixed value of 1V and sets this code. More than likely, the new sensor isn't working properly because the internal resistance heater isn't functioning properly. Check the fuses in the underhood PDC (Both O2 sensor heaters are fed by a single 15A fuse) to make sure they're all working, you should also take a look at the ground ring connectors mounted to a stud on the passenger side block adjacent to the oil dipstick. Finally, take a good look at the wiring harness running along the trans tunnel to the downstream O2 sensor, it's common for the harness to become damaged from contact with the driveshaft.

-Ed Rico
agreed. the P0138 code for my 96' was a pain (search for my posts a while back), but yes, the rear sensor or wiring has something wrong. either that...or...your cat converter is worn out and the PCM is not seeing any difference between upstream (pre cat ) and after cat (rear oxy sensor behind converter) oxy signal.
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Unread 01-22-2009, 04:55 PM   #6
Ed Rico
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A bad or gutted cat should set the P0420 Cat efficiency below threshhold code, this will indicate that the O2 content of the downstream sensor is the same as the upstream sensor, with a small time delay. You may well have a bad cat, but you won't know until you get an actual voltage signal from the rear sensor so the PCM can compare it to the front sensor output.

-Ed Rico
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Unread 01-22-2009, 06:13 PM   #7
AZ Jeff
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Originally Posted by Ed Rico View Post
A bad or gutted cat should set the P0420 Cat efficiency below threshhold code, this will indicate that the O2 content of the downstream sensor is the same as the upstream sensor, with a small time delay. You may well have a bad cat, but you won't know until you get an actual voltage signal from the rear sensor so the PCM can compare it to the front sensor output.

-Ed Rico
GOOD DIAGNOSTICS, Ed. I have to admit I did not know all the subtleties here until you spoke up.
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Unread 01-23-2009, 06:11 AM   #8
Ed Rico
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Thanks Jeff. Lots of good descriptions on these codes at http://www.obd-codes.com. Between the FSM and the code descriptions, you can get a better feel for what's going on when the Check Engine light comes on. The P0138 high-voltage code on the downstream sensor was a big headache for me on my '97, it didn't make any sense to me that the sensor would read 'high' if it wasn't working. I verified by scanner that the rear O2 sensor voltage output was a constant 1V, which converted to the expected variable voltage output once I figured out that I had a bad ground on my O2 sensor heater circuit.

Moral of the story is:
  • OBD-II codes on '96-up provide lots of insight and you should always pull them if the Check Engine light comes on.
  • Even without a Check Engine light, if you suspect a problem, go ahead and scan anyway, you may have a 'soft code' that didn't set the light
  • As always, there is no substitute for understanding the fundamental theory of system operation behind the error code. As others have stated, the OBD-II code just tells you what the PCM thinks is wrong, it doesn't necessarily tell you *why* something's wrong. Lots of good information in the FSM, it's worthwhile to spend some time going through it when you have a problem.


-Ed Rico
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Unread 01-23-2009, 06:36 AM   #9
tjwalker
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Ed, that is a great link you provided. Everybody should bookmark it. Eventually, you'll appreciate the help the information that website can provide. BTW Ed, I have noticed that you have become the go-to guy for code/oxygen sensor issues.

As you mentioned, the OBD systems can be very helpful in isolating problems. If/when a check engine light appears, ALWAYS pull the code as your starting point. Guessing is futile and throwing parts at a problem makes no sense. You need a fundamental, methodical strategy and that involves using the technology (OBD) your Jeep provides to you.

Not always straight forward though. Diagnosing problems with sensors can easily overlap other management systems because of the inter-relationships of the components. That is where the understanding of the fundamental theory of the systems comes in.
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Unread 01-24-2009, 05:22 PM   #10
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Originally Posted by tjwalker View Post
Ed, that is a great link you provided. Everybody should bookmark it. Eventually, you'll appreciate the help the information that website can provide. BTW Ed, I have noticed that you have become the go-to guy for code/oxygen sensor issues.

As you mentioned, the OBD systems can be very helpful in isolating problems. If/when a check engine light appears, ALWAYS pull the code as your starting point. Guessing is futile and throwing parts at a problem makes no sense. You need a fundamental, methodical strategy and that involves using the technology (OBD) your Jeep provides to you.

Not always straight forward though. Diagnosing problems with sensors can easily overlap other management systems because of the inter-relationships of the components. That is where the understanding of the fundamental theory of the systems comes in.

TJ very well stated, to many backyard mechanics and pros for that matter will pull a code and go out and buy the cheapest part they can and swap it out without putting some thought into the underlaying prob. And furthermore dont expect some snot nose kid behind a counter at some big chain parts store to diaganose your MIL light. Ya sure sometimes they get it rite but its your money they are playing with....Rob
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Unread 01-24-2009, 07:06 PM   #11
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from my oct 08' P0138 problem...that was actually the front oxy sensor. Here's why:

P0138 is the after-cat oxy sensor under the generic OBD code.


in my 96, the OBD II rear oxy sensor signal was "piggybacked" with software by chrysler under court order for all 4.0's XJ, (mandatory new cat and eprom flash at dealership before you can renew smog, or prove new cat and still have to get the flash, as in my case) so that when either oxy sensor started getting "lazy", it eventually triggered the P0138 code. That supposedly is the rear oxy sensor...It ended up being the front oxy sensor. Additional info is my cat was new, and the rear oxy sensor was also a new, top-quality, non-Bosch unit.

I certainly did not think that the P0138 code was the front oxy sensor...but that's what it was...

Easy fix. you're gonna be out around 50-80$ but getter mpg doing it.
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Unread 01-24-2009, 09:35 PM   #12
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not to hijack or anything, but my problem is in the same vein. did a complete tune up few months ago (cap, rotor, plugs, wires, oil change, cleaned air filter, cleaned throttle body, changed cat, changed O2 sensors) and 2 days after i changed everything, CEL came on. Pulled the codes and this is what I got:
PO132-O2 Sensor High Volts Sensor 1
PO135-O2 Sensor Heater Circuit 1 Malfunction
PO138-O2 Sensor High Volts Sensor 2
PO141-O2 Sensor Heater Circuit 2 Malfunction
Annoying the heck out of me because I've cleared the codes, run it for a day or two, and then they come back on. I've done a visual inspection and everything seems fine. Any of you more experienced guys have any insight or tips on where to start checking for problems? I know there's nothing seriously wrong because everything is running smooth, but I've noticed that my mpg's seem to be lower on average, and the light is just flat out annoying. Thanks

BTW, the O2 sensors are Bosch if that makes a difference. If that might be the problem, what's a reliable brand of O2 sensors to go with?
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Last edited by BayouBengal85; 01-24-2009 at 09:40 PM.. Reason: additional information
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Unread 01-24-2009, 09:36 PM   #13
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are you sure its not the sending unit code?
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Unread 01-25-2009, 05:25 AM   #14
tjwalker
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Quote:
Originally Posted by BayouBengal85 View Post
Pulled the codes and this is what I got:

PO132-O2 Sensor High Volts Sensor 1
PO135-O2 Sensor Heater Circuit 1 Malfunction
PO138-O2 Sensor High Volts Sensor 2
PO141-O2 Sensor Heater Circuit 2 Malfunction
The heater circuit is fused. Have you've checked the 15A fuse underhood in the PDC?
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Unread 01-25-2009, 05:31 AM   #15
Ed Rico
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Quote:
Originally Posted by BayouBengal85 View Post
not to hijack or anything, but my problem is in the same vein. did a complete tune up few months ago (cap, rotor, plugs, wires, oil change, cleaned air filter, cleaned throttle body, changed cat, changed O2 sensors) and 2 days after i changed everything, CEL came on. Pulled the codes and this is what I got:
PO132-O2 Sensor High Volts Sensor 1
PO135-O2 Sensor Heater Circuit 1 Malfunction
PO138-O2 Sensor High Volts Sensor 2
PO141-O2 Sensor Heater Circuit 2 Malfunction
Annoying the heck out of me because I've cleared the codes, run it for a day or two, and then they come back on. I've done a visual inspection and everything seems fine. Any of you more experienced guys have any insight or tips on where to start checking for problems? I know there's nothing seriously wrong because everything is running smooth, but I've noticed that my mpg's seem to be lower on average, and the light is just flat out annoying. Thanks

BTW, the O2 sensors are Bosch if that makes a difference. If that might be the problem, what's a reliable brand of O2 sensors to go with?
Same advice given above applies here also, but in your case both upstream and downstream O2 sensors are not generating a variable voltage output signal in response to exhaust stream O2 content and the PCM substitutes a fixed voltage output and sets the high voltage codes. The O2 sensors require an operating temp of approximately 600F, that's why they use internal resistance heaters to quickly achieve and then maintain operating temp. Without a functioning upstream O2 sensor, you're stuck in open loop and will eventually destroy your cat due to the excessive fuel content in the exhaust. Start with checking the underhood PDC fuses, there is a single 15A fuse that supplies both upstream and downstream O2 sensor heaters in parallel. Make sure the wiring harnesses to both sensors are intact, especially the downstream sensor, which is vulnerable to damage from the rear driveshaft. Don't forget the ground ring connectors attached adjacent to the dipstick. Finally, Bosch O2 sensors have a good reputation but if you want to check each sensor heater for internal shorts or opens, the 2 white wires on each sensor are for the heater.

-Ed Rico

PS If you do check the resistance of the O2 sensor heaters with a meter, be careful not to probe the black and gray wires, which are the voltage signal output and ground wires. In order to check resistance, the meter applies a voltage to the circuit, which you should never do to the signal side of an O2 sensor.

Last edited by Ed Rico; 01-25-2009 at 05:43 AM.. Reason: PS added
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