Hunting down lean condition P0139 Honda Fit 07

[ltslimjim]

So, I did the drive test WOT shows .8 V and higher and DFCO shows under .2 V and flatlines to .000 within 2-3 seconds max.

Question: Is it normal for speeds under 40 MPH when the torque converter is no longer locked up for the ST fuel trim to show the computer adding fuel even while no throttle is applied? Or is that the engine keeping itself from staling when the RPMs drop; no longer able to keep fuel cut off?Seems like common sense.

So, it appears as though the rear 02 is working fine. I’ll post up the Freeze Frame data next time the code trips.

I suppose the car just isn’t as efficient these days. It’s just strange to me that light throttle ST FT has the computer request 7-16% fuel and that isn’t considered excessive? Is that a symptom of fuel system lagging behind? It doesn’t appear to be a problem at WOT.

 

[dbias]

Could the fuel dump into the cat have caused heat damage to the wiring going to the O2 sensors?

 

[ltslimjim]

Could the fuel dump into the cat have caused heat damage to the wiring going to the O2 sensors?

The wiring harness from the computer is located inside the center console and doesn’t leave the vehicle. Instead, after the sensor connects there it goes under the vehicle and that section has a grommet for plugging the cab from the outside elements.

I mean, is that sort of thing common?

I’ll grab some freeze frame data next time it trips this code.

It’s possible the supposed need to add extra fuel is normal and unrelated.

 

[Luftmasse]

@Luftmasse my reasoning was perhaps a leak before the Upstream A/F ratio sensor, causing a rich condition by thinking it needs to add fuel because the computer sees lean due to additional oxygen. That was the theory anyway. In that scenario, would the rear 02 throw a P0139 because of poor running of the engine? Based on my general reading and limited understanding, a bad fuel injector or something with throttle can stop the car from entering DFCO.

Would a poorly positioned rear 02 cause the readings of 10-15% additional fuel requested by the computer on light to moderate throttle? I thought those readings would be determined by the Upstream A/F ratio sensor. Or is that pointing to a fuel delivery issue and a separate issue from the P0139?

Getting more photos.

We are trying to diagnosis. why is the rear o2 sensor is being lazy.

Technically, a good oxygen sensor should show activity from lean to rich (100 mV to 900 mV) under 100 mS. Since you don't have a scope, we can only work with what you have.

And yes, that aftermarket catalyst is not correct for your application. The correct location of the rear 02 sensor should be after the catalyst originally; think we May have found the culprit.

 

[CatCam2]

Just throwing this long shot out there -- could your MAF be messing with your fuel mixtures? I had something similar (not exact) happing to a chevy silverado - $8 can of MAF cleaner and done. Like I said, long shot but worth a look.

 

[ltslimjim]

We are trying to diagnosis. why is the rear o2 sensor is being lazy.

Technically, a good oxygen sensor should show activity from lean to rich (100 mV to 900 mV) under 100 mS. Since you don't have a scope, we can only work with what you have.

And yes, that aftermarket catalyst is not correct for your application. The correct location of the rear 02 sensor should be after the catalyst originally; think we May have found the culprit.

I don’t think that’s accurate. The OE clearly has a mid-catalyst “Downstream” 02 from what I can find online. On Honda part websites, it’s even called a “Middle Oxyen Sensor”.

Note part 16 and part 3. The location of the sensor is correct.

 

[ltslimjim]

Just throwing this long shot out there -- could your MAF be messing with your fuel mixtures? I had something similar (not exact) happing to a chevy silverado - $8 can of MAF cleaner and done. Like I said, long shot but worth a look.

This engine is equipped with a MAP sensor rather than a MAF. Thanks for the suggestion tho!

 

[Luftmasse]

I don’t think that’s accurate. The OE clearly has a mid-catalyst “Downstream” 02 from what I can find online. On Honda part websites, it’s even called a “Middle Oxyen Sensor”.

View attachment 100328

Note part 16 and part 3. The location of the sensor is correct.

Interesting, I actually called the dealer they had informed me it is after the catalyst. I have also have seen these vehicles with rear o2 sensor behind the catalyst in person also.

Assuming the picture above holds true, test the o2 sensor as I asked to see if it is really lazy.

 

[ltslimjim]

Interesting, I actually called the dealer they had informed me it is after the catalyst. I have also have seen these vehicles with rear o2 sensor behind the catalyst in person also.

Assuming the picture above holds true, test the o2 sensor as I asked to see if it is really lazy.

The full throttle followed by DFCO after? I think I mentioned that earlier (SEE post #21). It reads 0.8 V (800mV) & higher on full throttle and within 1-2 seconds after letting off throttle it goes under .2 V and flat lines to 0.00.

I also found a couple YT videos that confirm mid-catalyst 02.

 

[Luftmasse]

The full throttle followed by DFCO after? I think I mentioned that earlier (SEE post #21). It reads 0.8 V (800mV) & higher on full throttle and within 1-2 seconds after letting off throttle it goes under .2 V and flat lines to 0.00.

I also found a couple YT videos that confirm mid-catalyst 02.

Lean to Rich should be under 100 milliseconds. Longer than that would be considered a lazy o2 sensor.

 

[ltslimjim]

Lean to Rich should be under 100 milliseconds. Longer than that would be considered a lazy o2 sensor.

Is that for full throttle or fuel cut off after throttle? It seems fairly quick and remember I’m using a $30 cheap scanner that isn’t as fast in of itself. This could be adding some lag.

WOT is immediately rich. Fuel cut off kicks in after letting off throttle and the reading goes to 0.00.

 

[Luftmasse]

Is that for full throttle or fuel cut off after throttle? It seems fairly quick and remember I’m using a $30 cheap scanner that isn’t as fast in of itself. This could be adding some lag.

WOT is immediately rich. Fuel cut off kicks in after letting off throttle and the reading goes to 0.00.

You need a scope to measure the timeframe from transitioning from Rich to Lean to the next Rich. Since you do not, we can only assume it is good from your statement above. Intermittent problems are hard to solve.

 

[slacktide_bitog]

Interesting, I actually called the dealer they had informed me it is after the catalyst. I have also have seen these vehicles with rear o2 sensor behind the catalyst in person also.

Assuming the picture above holds true, test the o2 sensor as I asked to see if it is really lazy.

It's possible that the CA emissions has it in the middle, while federal has it after the cat

 

[ltslimjim]

It's possible that the CA emissions has it in the middle, while federal has it after the cat

Maybe. But if anything, the opposite would be true since I’ve dealt with 2 FED in person and seen a couple online with the middle version.

I’ve also seen a European Jazz model on a video earlier with it post catalyst. Maybe that’s where this info is coming from? Here is a Jazz model:

You need a scope to measure the timeframe from transitioning from Rich to Lean to the next Rich. Since you do not, we can only assume it is good from your statement above. Intermittent problems are hard to solve.

Indeed so!

After watching one of the YT videos earlier, I went ahead and replaced a fuse in the passenger cab that had some corrosion on it and it’s related to several things with the vehicle including the 02. Sometimes a P0141 is thrown related to this fuse. Cheap to throw a new one in there and rule it out!

Reference video:



Might be important: There is a water leak into the rear hatch and it sits in the spare tire well.

So, the interior is exposed to excessive humidity after rain and I live in Georgia where it’s humid almost year round.

To give you an idea, a few years ago the HVAC blower stops working because the fuse has gotten corroded but didn’t break!

Thanks again for the help.

 

[Luftmasse]

It's possible that the CA emissions has it in the middle, while federal has it after the cat

I probably got mixed up with newer model than his. But the diagram looks accurate.

 

[ltslimjim]

I probably got mixed up with newer model than his. But the diagram looks accurate.

The voltage at idle changes at least 3 times every second on this cheap scan tool I have.

I think without better tools, I could just wait for the next code and post the freeze frame data.

If the sensor is good and the location is accurate, what other possibilities? I’ve seen stuck open throttle but I don’t see that as the case here. The sensor might be a tad slow. Maybe an injector is faulty otherwise.

Is my understanding of the ST FT correct? That to add 15% it’s not enough to trip a CEL but it’s enough to signal the computer to add more fuel because it sees a lean condition? Only adds that amount on light to moderate throttle. More reasonable readings at idle and at WOT.

 

[Luftmasse]

The voltage at idle changes at least 3 times every second on this cheap scan tool I have.

I think without better tools, I could just wait for the next code and post the freeze frame data.

If the sensor is good and the location is accurate, what other possibilities? I’ve seen stuck open throttle but I don’t see that as the case here. The sensor might be a tad slow. Maybe an injector is faulty otherwise.

Is my understanding of the ST FT correct? That to add 15% it’s not enough to trip a CEL but it’s enough to signal the computer to add more fuel because it sees a lean condition? Only adds that amount on light to moderate throttle. More reasonable readings at idle and at WOT.

The code P0139 relates to a connection problem or a sensor problem.

When the fuel adjustment reaches to a certain threshold, it will trigger the CEL. Although 15% may sound a lot to you, you need to consider the total fuel trim (STFT and LTFT).

 

[ltslimjim]

The code P0139 relates to a connection problem or a sensor problem.

When the fuel adjustment reaches to a certain threshold, it will trigger the CEL. Although 15% may sound a lot to you, you need to consider the total fuel trim (STFT and LTFT).

I see. I am having decreased fuel economy; about 4-5 MPG less. I used to get 31-32 MPG on a tank and now I struggle to get 28-29. Sometimes as low as 26.

A couple of google results referenced other possibilities for a P0139. And that while what you said is most likely or usually the cause, they list the following as well:

Fuel pressure issue
Faulty Injector
Intake air leak
Exhaust gas leak

I should add I was reminded by my BIL today that when we replaced this Downstream 02, the old one that was migrated over from the failed CAT by the shop; well it had stripped threads and it made installing the replacement 02 difficult. This was to the point that if we need to replace this 02, it means a new CAT probably lol (since I don’t think the threads can be salvaged!)

It was able to be fully installed, but the thought of throwing another 02 at it. I may just wait until this aftermarket fails. If I get 5 years out of it, I’ll be happy.

 

[ltslimjim]

@Luftmasse So, I made a video with a graph of live data. Please ignore my rambling speculation.

 

[Luftmasse]

@Luftmasse So, I made a video with a graph of live data. Please ignore my rambling speculation.

Not sure about your fuel economy but if the sum of STFT and LTFT is under 10%, I would say you don't have a lean condition or injector problem.

Maybe try wiggling the o2 sensor harness and see a drop out on the signal? If not, connection is fine and that is one off the list.

If all checks out fine, you do not have an exhaust leak, I would just replace the sensor with an OE o2 sensor. There are times aftermarket parts are not reliable. Not much to say for a code like this.