Help with O2 codes

[PapaSmurf]

(Reposting this in the KJ forum - inadvertently posted in KK originally)

Bought my KJ used 2 weeks ago. Within hours of the purchase got a CEL. Checked the codes a couple of days later and had both downstream O2 sensors showing high voltage (low? can't remember).

About a week later purchased a scanner and cleared the codes. Drove about 10 minutes and CEL back on - same issues.

Check the codes this evening and now have four codes P0138, P0139, P0158, and P0159. From what I am reading both downstream sensors are showing "high voltage" as well as "slow response."

I have searched here and overall and get the concept that this may be more than just the O2 sensor. Just not really sure how to go about doing a good diagnosis. My thoughts are to simply replace the sensor and see what happens (I have over 106+ miles on it and it may very likely be time for them to be replaced - don't know the maint history of the vehicle). But at the same time I hate to simply waste the $$ on a new part if that isn't the problem.

Saw a video where someone was checking a Nissan and did an voltage check on the harness. What voltages should I expect to see in the wiring?

Any help will be appreciated. I'm not a mechanic but have a generalized understanding of doing basic stuff.

 

[JasonJ]

Depending on the code scanner/reader you bought, some can show you real-time voltages of all of the O2 sensors..

I've used this feature to watch the voltage graphs of the sensors, and you'll be able to easily see slow responses, or voltage readings way out of whack.

Most, if not all sensors like that use 5v referance voltage from the ECM.. then do their thing and convert it back out as a signal voltage. O2's operate with output voltages between 0.1v and 1.0v. As the oxygen readings change, and air/fuel ratios are altered, the sensor rapidly moves up and down within this range.

If it's pegged at a high voltage, or seems to be hovering in the upper range, without lots of movement back down to a lower voltage then back up again, it can be indicative of two things. One is that the sensor is bad, especially if you are also seeing the "slow response" code and can visually verify that on the voltage graph.

The other is that the high output voltage (anything at or near 1.0v) indicates a consistently rich air/fuel mix.

Typically, if the engine is running so rich as to cause both O2's to peg at a high voltage, you'd expect to see the engine running poorly as well, and the sometimes, you also see MAF sensor or MAP sensor faults, fuel trim codes, misfires, etc.

If the engine seems to run strong, no black smoke out the tailpipe, no other DTC's, I'd be inclined to think that the O2 sensors are faulty. They shouldn't be too expensive at all, maybe $25-30 each from a local aftermarket parts store. I know up here at my local store they're around $25 each. I am uncertain if the Liberty is one of those cars that only does well with OE branded O2's, like NTK, or similar, of if they do well with Bosch and other aftermarket brands. Worth looking into.

If you can't see voltage outputs with your scanner, you can connect a digital multimeter (DVOM) to the output wires and monitor the voltages that way. Be cautious though, you'll need very long leads, likely some test probes/pins and 15-20' test leads. Run the leads away from the exhaust, as you'll need the engine running to test this and don't want to melt your test probes.

Good luck, I'll check back here to see what you've found out.

 

[PapaSmurf]

Ok, so I bought a reader that does live display. Actually pretty neat.

Following is a link to a small video I made showing my O2 Sensor voltages in action over about a 90 second period. Not sure if this is enough to actually tell anything or not. https://youtu.be/eSTcXhnQnfo

I will say that I have seen this vary. Last night I was doing some checking and the S2 voltages were both over 1.x. Today it seems that they were all under 1.0 for the most part.

No matter, every time I clear the DTC's the CEL comes back on within a couple of miles.

If I need longer time or while moving I can probably do that as well.

 

[JasonJ]

I could get a pretty good idea of what they were doing after just 30 seconds. I didnt have audio turned up (at work) but I presume that was at idle?

If you modulate the throttle while looking at those voltages, they should change more rapidly or at least change at all.

I could tell that both of your S2 sensors didnt move much, if at all, and when they did it was quite slow. Compare that to both of your upstream (S1) sensors and it's quite different.

This aligns with the fault codes you are seeing.

P0138 B1S2 High circuit voltage
P0139 B1S2 Slow response
P0158 B2S2 High circuit voltage
P0159 B2S2 Slow response

Certainly, there are other things that could cause that, but those codes, specifically, and returning exactly every time... bud I'd just drop the $50 on two new sensors, clear the faults and see what happens.

If you'd like to do further testing first, start by testing the O2 circuit grounds. Make sure that the O2's are properly grounded. I dont have the "approved" Jeep/Chrysler testing methodology for this, but if it were me I'd probe my O2 ground wire (at each sensor, one at a time) and set my DVOM to continuity... put the other probe on the engine block or known good grounding source. If you have continuity, sensor wire is properly grounded.

O2's will usually, but not always fail in a rich-indicating direction, high voltage as opposed to seeing them crap out with a low voltage code. So that further reinforces a pair of faulty downstream O2's in your case.

Not sure if you have an EGR valve on your 04... probably not, but if you do, check it to see if it's stuck open. It should throw a code for that, but not always. An EGR stuck open will cause O2's to read high. Although you'd also expect to see your upstream sensors freaking out about that as well, since all of your codes are for downstream, this is less likely.

It is also possible that both your Bank 1 and Bank 2 (left and right side) catalytic converters are not working properly and setting this fault codes. There are specific DTC's however for low catalyst efficiency, so the lack of those codes makes this lower on the likely list, but don't count it out 100%.

On most Jeep/Chryslers, if there is an open circuit fault (which could throw a code, or be indicative from your high voltage code), then the voltage wire for the O2's will be stuck somewhere between 2-4v. Even though this is not indicated on your scan tool, I'd check it manually with a DVOM and verify.

Also be sure to blip the throttle while checking voltages with the scan tool, note the vMIN and vMAX values.. the lowest and highest voltages the sensors read.. if they look normal.. and are on either side of the voltage spectrum, that is good.. if it doesnt change much, or they are super close together.. that can indicate and further reinforce a faulty sensor.

After all of this, you'll have to decide if you have a grounding issue, a circuit/wiring problem to the rear O2's, or simply two bad sensors and some parts to replace.

Good luck.

 

[PapaSmurf]

Well it's been awhile. Purchased 2 rear O2 sensors and tried to get them in. But for a variety of reasons (coupled between weakness and the rust on the exhaust system) I could not get them in. Took the KJ in for some work and had my local mechanic install the O2 sensors.

As expected the check engine light has been extinguished ever sense. Much thanks to JasonJ for helping with the diagnosis.