About two weeks ago my check engine light came on. I have a '96 GC Laredo with a 4.0 engine. I went and had Autozone check the code and it was PO138. Got the Haynes book out and it said it was the o2 sensor before the cat. I replaced it and drove for a few days and the light came back on, went to Autozone and it was the same code. It's happened three times since I replaced it. Any suggestion as to why it keeps coming back?
An engine code tells you that there is a problem in that particular circuit. It doesn't pinpoint the component itself as the cause. Since you have pretty much eliminated the O2 sensor itself, then there is probably a problem in the wiring or a connector. Unless you got a sensor that is bad out of the box - unlikely but possible, I guess.
You need to inspect the wire harness under the Jeep to see if it is damaged (burned or crushed), and maybe get out the VOM and do some continuity tests between the ECU and the O2 sensor connector. This might be a good one for a certified tech at the dealer.
Good luck on this one. :(
John Davies
Codes don't mean diddly they just provide a starting point. You need to trouble shoot using the go-no-go part of the shop manual which could be quite lengthy. Codes are symptomatic so all you are doing is treating the symptoms not the problem. The only true way is to hook up the vehicle to a computer (OBI/II) and read the data stream that is the only true way to diagnose what your issue is.
I've been told a gas cap going bad or loose could cause this kinda thing.
L8r - Jersey
Are you sure that's an O2 sensor code? I thought that was a muffler bearing code. Have you replaced that yet?
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