lgrant

99 XJ 4.0L
Just Thur, Jeep began throwing P0138 (high voltage from downstream O2 sensor). I separated the connector and checked the resistance (ohms) of the heater circuit. There is continuity but no resistance at all - should be 4 to 7 ohms. What's our best guess as to what's happening: is the heater working harder than it should and therefore making the sensor circuit think it's at that lean stage to create 1.2+ volts for the required amount of time? I'm at a bit of a loss as to why P0141 wasn't thrown. Maybe that comes into play when the heater circuit isn't working at all?

It is my understanding that this DTC (P0138) is totally benign and that I can drive with this code, with no loss of performance or gas mileage, and take my time to fix it. All indications are that the sensor is bad and will order genuine mopar from one of the mopar discounters.

Thanks in advance for the help.
Larry

dave1123

If you have a scanner, check live stream data and see if it's cycling. It could be the heater is shorted to the sensor circuit.

97grand4.0

I would suspect you aren't reading the ohm meter right. you have to be on the very lowest scale to read the single digit ohms. That said, two things, cat is not working, or sensor is bad. my experience you end up changing both.

lgrant

Changed out the sensor with a Denso for $27 and all is good - for now. Factory sensor lasted 210K. We'll see how long the Denso lasts. If it craps out within 100k, I'll update.

97grand4.0

I have done 2 downstream, and from my experience they aren't so fussy as to the brand. Both times it cured the code.