97' Grand Cherokee Laredo Code P0138


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Old 05-03-2010, 12:15 PM   #1
Danno
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Default 97' Grand Cherokee Laredo Code P0138

Hi,
I have a O2 sensor problem. Code p0138 has been comming up intermitatly. So I put a Bosch OEM sensor in. Jeep dealer said "Oh No, only use our sensors". So the code was comming up all the time with the new Bosch sensor so I figured replace it with a Jeep sensor. So i did. The P0138 is still comming up with the Jeep sensor. I had a misfire code comming up in cylider 6 and fixed that with some Bosch platinum plugs when the 1st sensor was going bad.

The only weird performance symptom is a high idle speed which is intermitant after highway speeds are maintained. (1800rpm 2100rpm in park or nuetral) This may be a seperate issue.

I have had alot of advice, like the fuel pressure regulator is bad, or replace the cat. convert., or you have a short on the sensor wiring. This is the sensor mounted on the back of the Cat. convert.

From what I understand the sensor in question is reading input voltage maintained above normal operating range over a certain time. So would this lead me to think it's faulty wiring? Can any one HELP?

Thanks, Danno


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Old 05-05-2010, 04:50 PM   #2
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I saw this and it all seems right. I checked the ground ring on the engine block, It looked good. I can't find the fuse for the O2 sensors in the PDC power distribution center. Does any one know where it is on a 1997 Grand Cherokee Laredo 4 liter?


OBD-II uses upstream and downstream O2 sensors and compares O2 content to verify if cat is working effectively. Air/fuel ratio is adjusted based only on the signal from the upstream O2 sensor. Both sensors require an operating temp of approx 600F to reliably produce a voltage signal so they are equipped with internal electric resistance heaters to achieve and maintain operating temp, this allows the engine to achieve closed loop operation as quickly as possible. Without the expected voltage signal from the upstream sensor (as on cold startup), the engine runs off standard tables for air/fuel mixture, typically on the rich side. P0420 code indicates that signal from downstream sensor is similar to signal from upstream sensor and cat is not providing increased O2 content of exhaust stream as expected. P0138 is high voltage code from downstream O2 sensor, usually indicates that sensor isn't putting out normal varying voltage signal based on O2 content of exhaust stream after the cat, so PCM substitutes a fixed 1V. Typically caused by a bad O2 sensor, could also be caused by faulty internal heater or wiring to downstream O2 sensor.

Check the easy stuff first, both upstream and downstream O2 sensor heaters are supplied in parallel by a 15A fuse in the underhood PDC, you also want to check the ground ring connectors for the heater circuit located on a stud on the passenger side of the engine block adjacent to the dipstick bracket. If you don't find anything here, the sure way is to have the vehicle scanned with a device that can check the datastream to see what the actual voltage signals are from both upstream and downstream O2 sensors. If the internal resistance heaters and wiring harnesses to both sensors are intact and you still don't get a varying voltage signal, the O2 sensor is shot and needs replacement. I would focus on the downstream O2 sensor code first, the PCM can't actually determine if the cat works or not without a valid signal from both sensors. You should also check for internal rattling from the cat, a sure sign that the cat is shot.

Even without scanning the datastream, if both O2 sensors are the originals with 100K+ miles on them, they're due for replacement even without a specific code for the upstream one. From what I've seen, in the usual sequence of events the upstream sensor gets lazy or stops working completely, then the resulting overly rich fuel mixture over time takes out the cat and the downstream O2 sensor. So don't be surprised if you wind up needing a new cat as well as both O2 sensors. Best of luck to you.

-Ed Rico
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Old 05-27-2010, 03:51 AM   #3
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Default Fixed code P0138

Well this problem was all due to faulty wiring between the bank 1 sensor 2. (the one behind the cat convert.) Seems the previous owner did a poor job fixing damaged wiring going to the engine from the O2. Fixed at my local shop for $120.00. Thank you bob for being a good mechanic.
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Old 05-27-2010, 06:28 AM   #4
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Glad you got it fixed.

I see you are from Rochy. I went to RIT for a couple years there.


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