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?
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.
Im new to the forum but my jeep had similar problem but replaced o2 sensor with OEM and problem went away...stay away from after market o2 sensor is what my research found..
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.
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.
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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