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Which 02 Sensor Is Bad?

1K views 6 replies 7 participants last post by  Wheelman04 
#1 ·
Howdy,

I have a P0138 Code on my 99 WJ 4.0. Says it's an O2 sensor...possibly a bad fuse. Both O2 sensor fuses work.

I have a non-California emissions Jeep. The best I can tell from actually seeing and diagrams is there are 4 sensors on the Jeep. Two are located underneath where the exhaust pipes come out. And two are sort of near the top of the engine.

The sensors underneath have a different connection than the ones on top.

So, how in the world do I tell which sensor is the bad one? Is there a different fault code for the sensors on top?

Thanks for the help!

Chris
 
#3 ·
Before you even consider replacing a sensor, carefully inspect the wiring, particularly in areas near the exhaust manifold where temperature are the hottest. Wires are known to melt causing shorts in the wiring.
 
#5 ·
I had to replace an O2 sensor too and I read all the horror stories of aftermarket ones. But I took the gamble anyway figuring I can always buy a very expensive OEM one if I have to. Well, I haven't had to so far, the sensor still works a charm. So I guess it kinda depends on what aftermarket sensor you get.
 
#6 ·
I just got a p0138 code on my trip out to the scrap yard. Seems like it is the downstream sensor. I will check it out tomorrow at the shop. Guessing it might be a fried wire and not actually the sensor. I will be under the WJ anyway to install the front anti-sway bar that I picked up so that I actually have one.
 
#7 ·
This can also be a common issue if you have the 4.0 inline 6 cylinder the main harness that runs along firewall behind engine rubs on rear valve cover studs wearing through the harness creating a short. I had problem for months constantly getting O2 sensor codes even after replacing all sensors.. Definitely look into this...It's a well known flaw
 
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