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Old 03-02-2009, 10:59 AM   #1
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Rear O2 Sensor Issue

So I just picked up my 03 TJ and the check engine light is on. The code indicates that it's the rear O2 sensor. Replacing the sensor is no big deal but I'm wondering if the sensor is likely to be the root cause or just a symptom of some other issue. The Jeeps only got 56K on it so I'm surprised to see a failure of this sort. Are these sensors prone to failure? Has anyone replaced one and then had no other subsequent issues? Any info is helpful.

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Old 03-02-2009, 11:01 AM   #2
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ya, they're common failure. and if the code is specific for that oxygen sensor, then its the way to go. the computers these days are accurate on picking up unlikely or bad information recognizing the o2 being bad, compared to conditions causing problems such as lean or rich conditions.

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Old 03-02-2009, 06:01 PM   #3
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replaced my 02 sensor (bank 1 sensor 2) which is after cat a month ago.. only drove around 100 miles code came back.. and i cleared code thinking it was a read from the previous sensor.. it came back in 14 miles .. did that 4 more times i cant get the code to stay off... it may not be 02 sensor.. try cleaning the electrical connections
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Old 03-02-2009, 06:43 PM   #4
Knows a couple things...

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It's pretty tough for the computer to know for sure if the 02 sensor behind the cat is bad or if it's really just the cat the 02 sensor is monitoring that is bad. Cats do go bad.
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Old 03-02-2009, 06:52 PM   #5
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the computer uses both o2's to monitor the cat. the computer is pretty accurate, not fool proof, at picking up lazy or irrational readings from an 02. when the cat goes bad the o2's will stay around the same readings, not switching and the downstream sensor does not stay around half the upstream range. the computer sees when the front o2 is normal and the rear is lazy or out of range giving "irratrional readings". when the cat is bad it is easy for the computer to tell by both sensors. computers 420 catalyst efficiency below threshold is always dead nuts. a specific oxygen sensor code is usually reliable. it is difficult for the rear o2 to fail in such a manor mimicing a bad cat.

the only way to know for sure is testing it with a scan tool.
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Old 03-21-2009, 10:39 AM   #6
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I believe that (Bank 1 Sensor 2) is the oxygen sensor right before the cat after the "Y" off of the engine...
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Old 03-21-2009, 12:36 PM   #7
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no its right after... and i replaced the sensor.. code came back... its my cat...
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Old 03-21-2009, 04:11 PM   #8
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you are right, I was told the wrong info being that our 97's only have the two O2 sensors so bank1 sensor 2 would be the one behind the cat. The newer models have like 4 O2 sensors and that is what they were going on......
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Old 03-21-2009, 05:14 PM   #9
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time to get random!
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Old 05-21-2009, 10:21 AM   #10
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hey im just gettting out of school to be a tech and there are a lot of things to think about when a 02 sensor goes out. I would recomend if the code comes back on after the cat is replaced that you hook it up to a scan tool where you can look at the different sensor PIDS. This way you can see what the o2 sensor is doing to the different fuel trims and why its causing the code to be set. In a normal operation a cat should not fail, but when the engine runs to rich or something is leaking into (aka antifreeze) the engine, this causes them to fail. I would make sure before you replace the cat you find out why it went bad otherwise you could be replacing it again down the road. Make sure the scan tool has a genertic function that way you can look at the different mode test i think it is mode 6 or 9 that is the cat effiency test, again you need to have a scan tool to do this.
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Old 05-25-2009, 12:19 PM   #11
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I had a similar deal with my 99 . It said the same rear 02 sensor which I replaced along with a partially melted plug and eventually soldiered the wires directly only to find out the previous owner had messed up a wire by the front 02 which I think had something to do with both sensors and a few others . I soldiererd that wire and shrink wrapped it and its been fine for 6 months now .

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