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Unread 10-16-2013, 11:25 PM   #1
Registered User
2005 TJ Wrangler 
Join Date: Oct 2013
Location: Buffalo, NY
Posts: 97
O2 Sensor Problem...

Hi i'm new here, but i'm curious if anyone can relate to the problem i'm having. I have a '99 Cherokee Limited with a straight 6. My check engine light has been on for code P0138 and i'm having a hard time narrowing down the exact issue. What it does is sometimes (probably half the time) when I start driving it will nose over and run real choppy. It does it mostly when the engine is still cold. Occasionally at idle (like at a stop light) the RPMs will drop and it will almost stall. And there has been a couple times while standing outside it (while it's running) you can smell it's running very rich and it will actually stall.

I've changed the plugs/wires and cap/rotor. And i've changed both O2 sensors. I did replace the downstream twice thinking one was defective but still didn't solve the problem. I was told by a knowledgeable source that the O2 sensors actually aren't working.... for whatever reason. I did have a problem awhile back where the wires were pulled directly out of the downstream O2, but the plug wasn't damaged. So I just replaced it (the O2 Sensor). Is there a chance when the wires were pulled out it shorted something?

I'm very much a novice and not mechanically inclined but starting to get frustrated... just hoping someone can narrow down the problem a bit for me. Or at least point me in the right direction. Thanks


mlautner03 is offline   Reply With Quote Quick reply to this message
Unread 10-17-2013, 01:16 AM   #2
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1995 ZJ 
Join Date: Dec 2010
Location: Norman, OK
Posts: 2,256
How many oxygen sensors do you have? I thought the 99+ 4.0's had 4. Are you sure you replaced the correct 02 sensor? P0138 is the bank 1 downstream voltage too high (H02S-12). If you have the 4 sensor system just make sure you are looking at the right one. If you had a scanner that could monitor live data you should be able to see what the sensor is reading when the failure is present. Oxygen sensors normally produce a voltage around 0.2-0.8 volts, the downstream sensors should produce a more steady reading usually around 0.4-0.6v while the upstreams should be constantly switching between 0.2-0.8v when running. I think that code gets triggered when the voltage exceeds approx 1.2v maybe a little less. 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.

Does this pic of the exhaust/sensors match what you have? If so the sensor you would be looking for is the #4-1/2 in the diagram.
jnicewan is offline   Reply With Quote Quick reply to this message
Unread 10-18-2013, 05:48 PM   #3
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1992 XJ Cherokee 
Join Date: Oct 2012
Location: umatilla, florida
Posts: 1,961
99 xj has 2 o2's
uberxj92 is offline   Reply With Quote Quick reply to this message

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