How many miles are on your oxygen sensors? P0138 is usually a failing downstream O2 sensor.

 

How many miles are on your oxygen sensors? P0138 is usually a failing downstream O2 sensor.

For all I know it could be original but that's beside the point did you rear the thread? I know the code is related to a bad O2 sensor but my data doesn't correlate that AND the code went away. I am trying to determine what the normal operating range of the O2 sensor should be which looks like anywhere between 0.1 and 0.9, I drove around monitoring it for we'll over an hour with the code active and it appeared to be operating normal.

What exactly is a high voltage value for the Jeep? Is it over 1.0v? I don't want to just throw parts at it no matter how cheap they are because I don't learn anything and that's simply bad business for your wallet. I'd rather diagnose and understand the issue at hand and then rectify it.

 

If you really want to learn the correct way to check things, you should be reading a genuine Factory Service Manual for your model year.

Here is some basic O2 sensor info:

A leaky exhaust system or leaky fuel injector(s), or bad compression or leaky valves, bad plugs, wires, cap, rotor, HV coil, and so on, or combination of these, can also cause a lean or rich condition that gives you high or low O2 sensor readings that are not the O2 sensors fault, so try and verify those other items also before buying parts like an O2 sensor to fix your problem.

1991-2001 HO

For the HO years, 91-02, the O2 sensor has 4 wires. One wire is the 12-14 volt power that comes from the O2 sensor heater relay on the passenger side firewall (unless they moved them in later years to the relay & fuse boxes?), and that powers the internal heater in the sensor so that the sensor can work at idle, and almost immediately after start up. Loss of that power will hurt gas mileage even with a good O2 sensor.

Two black wires are grounds for the heater power and O2 signal to the ECU. A poor ground will limit current to the O2 sensors heater or cause an error in the O2 sensor output voltage read by the PCM (ECU) causing poor mileage even with a good O2 sensor. Test the 2 ground wires with power off. Test between the wire end at the sensor and the battery negative ground post. It should read less than 1 ohm.

The last wire, #4, is a signal feed wire, 0-1 volts, from the O2 sensor to the PCM (ECU) sensor. The O2 sensor is an O2 concentration sensitive variable voltage generator. At optimal O2 concentration the O2 sensor puts out 0.45 volts.

At idle that voltage should read 0.1-0.9 volts oscillating quickly back and forth roughly once every second. At 2000 rpm it should run between 0.4 and 0.5 volts max (in park). A digital meter can NOT be used for reading the O2 sensor voltage, but it can be used to test the grounds and the 12-14 volts to the heater. You must use an old style analog meter with the needle gauge on the display to see the voltage swing back and forth.

If the O2 sensor readings are not right, say they read .1 volts or .8 volts steady, you have a problem. BUT before you blame the O2 sensor make sure it has good wiring, and make sure the proper voltage is feeding it, by turning power on, engine off to read the engine off voltage feeds (12-14 on one, and ensure the ground wires (power off) reads less than 1 ohm to the battery negative post.

If any one of the grounds or the O2 sensor to ECU/PCM wires reads a high voltage chances are the wiring harness is damaged and the 12-14 volts for the O2 sensor heater is getting through a bad spot in the wires insulation and contacting a ground wire or the O2 sensor wire feed to the ECU/PCM. This is not the O2 sensors fault, but a wiring problem.

 

So I did some googling and it seems 0.1-0.9 is normal range of O2 sensor voltage
0.45 is theoretically perfect stoich and any number above is rich and any number below is lean
This isn't actually air/fuel readings since it's narrow band
But I have access to injector pulse on/off times and fuel trim percentages for corrections
Base line should be 0.0% and my short term fuel trim is 0.0% when idling and plus or minus 10% is within acceptable range for a normal running engine
My long term fuel trim is at 11% at idle and 6.5% at 30% load which for my purposes still within normal parameters since the CEL doesn't come on until 15%
So it seems my O2 sensors are reporting correctly and there is no error BUT why the CEL?

 

If you really want to learn the correct way to check things, you should be reading a genuine Factory Service Manual for your model year.

Here is some basic O2 sensor info:

1991-2001 HO <snip>

If any one of the grounds or the O2 sensor to ECU/PCM wires reads a high voltage chances are the wiring harness is damaged and the 12-14 volts for the O2 sensor heater is getting through a bad spot in the wires insulation and contacting a ground wire or the O2 sensor wire feed to the ECU/PCM. This is not the O2 sensors fault, but a wiring problem.

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So if you review my above post it seems that my engine is running correctly so I think this would be the avenue I need to pursue for seeing why the CEL popped up.

 

That, or this: A leaky exhaust system or leaky fuel injector(s), or bad compression or leaky valves, bad plugs, wires, cap, rotor, HV coil, and so on, or combination of these, can also cause a lean or rich condition that gives you high or low O2 sensor readings that are not the O2 sensors fault, so try and verify those other items also before buying parts like an O2 sensor to fix your problem.


I believe in preventative maintenance, meaning replacing parts before they fail, simply based on age or mileage. Typical O2 Sensor service life is about
100 - 150,00 miles.

 

I'm leaning toward the harness having a ground like you stated in the quotes above because all was fine until my down stream O2 sensor became unplugged a few days ago. But it wouldn't hurt to replace the O2 sensors either since they're possibly OE parts. However I'm going to follow the instructions above before I replace them.

 

So someone mentioned a Genuine Jeep Service Manual. I have tried to find on for purchase, 2011 Jeep Liberty Sport(KK) 4WD, throwing a P-300 code, common code I have found out, 2 possible fixes, plugs or o2 sensors. I would love to have a service manual but can't find one, ideas.