Troubleshooting P0171 Honda Fit

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hi guys, looking to hunt down a supposed lean issue on my 2007 Honda Fit.

Earlier this year, I had a pretty bad misfire from a coil problem driving to work and this eventually wound up causing the cat to fail afterward. Come to find out, this vehicle had an aftermarket cat from a PO!

Anyway, I replaced the cat because it was beginning to cause a restriction while breaking apart internally; I had this verified by a mechanic. I had the mechanic weld in an aftermarket MagnaFlow universal; due to previous owner exhaust mod which made a direct fit no longer a straight forward option anyway.

Just before replacing the cat; which had thrown a P0420 a number of times driving around with it rattling, well I also had a P0171 general lean code come up.

fast forward, I started looking at some live data with a cheap scan tool. Even though the cat replacement has kept the P0420 away and so far the P0171 has stayed off... well, I’m still getting poor fuel economy and very lean running at highway speeds; which was when I got the P0171 originally.

I’m uploading a screenshot of some data that shows the symptom while driving. It happens more at highway speeds; as the code tripped while going 74 originally... but sometimes it will stay stuck reading this way while coming to a stop after but will quickly go away. it hasn’t read this way at idle on its own so I have not suspected a vacuum leak.

If I’m reading this right, I believe the front/upstream AF sensor is reading very lean but the downstream is reading very rich:

4FEA463C-4ED1-450D-8331-9A22D5D1DE39.jpeg


Do I have a fixed lean upstream sensor? Of course it’s perhaps Instead an air leak somewhere or even a fuel delivery problem.

My understanding is a 4-wire AF sensor for a Honda shows negative current when lean and positive when rich; similar to a typical 02. Where as a Toyota 4-wire will read the opposite.

what will happen is if I slow down some during my commute; after it has read very lean at highway speeds, then it will read very rich from time to time and finally pull fuel and this leads to hesitation when trying to regain speed afterward; such as dropping cruise control to slow down some.

Otherwise, the car drives mostly fine and it doesn’t seem to have trouble at idle at least.

the good news: my BIL has the same vehicle and I will swap upstream sensors after I rule out some easier possibilities like ensuring vacuum Isn’t a factor or the EGR isn’t contributing. Fuel economy has dropped to barely 25MPG from usually over 30 without blinking.

Maybe a fuel delivery instead, but I think the screenshot points to an obvious discrepancy between the sensors.

side note: this engine has a MAP sensor, not the typical MAF used more recently.
 
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Honda, Like Chrysler isn't to big on using MAF sensors. Unmetered intake air leaks don't normally cause high fuel trim numbers at idle on Speed Density systems....Within reason. You can usually catch vacuum leaks with low IAC counts/low commanded throttle blade angle.

The upstream A/F sensor should have been replaced with the catalyst anyway, That's just due diligence!
 
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Any fuel trims should be within +/-10%, +/-15% at worst.
When is the picture taken?

Downstream seems ok.

I would replace upstream O2 sensor. Its cheap <$100, easy to do.
 

ltslimjim

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Any fuel trims should be within +/-10%, +/-15% at worst.
When is the picture taken?

Downstream seems ok.

I would replace upstream O2 sensor. Its cheap <$100, easy to do.
The picture is taken in this particular instance after coming to a stop before the numbers went back to normal.

Usually, this set of data does not show up at idle and only at speed.
Honda, Like Chrysler isn't to big on using MAF sensors. Unmetered intake air leaks don't normally cause high fuel trim numbers at idle on Speed Density systems....Within reason. You can usually catch vacuum leaks with low IAC counts/low commanded throttle blade angle.

The upstream A/F sensor should have been replaced with the catalyst anyway, That's just due diligence!

I guess the mechanic did not see any codes and figured it was OK? I mean, I do suspect now that the sensor was damaged as it seems lazy and inaccurate. I could not find information online that said to just replace it anytime a cat is replaced.

thank you both!
 
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do some research on air leaks.... many p0171 are caused by unmetered air entering the cylinders. look at all tubing between maf and throttle body fix all leaks can you access freeze frame data ?? oops speed density but the same rationale applies
 

ltslimjim

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do some research on air leaks.... many p0171 are caused by unmetered air entering the cylinders. look at all tubing between maf and throttle body fix all leaks can you access freeze frame data ?? oops speed density but the same rationale applies

Wouldn’t a vacuum leak happen more so at idle rather than at speed? Unless, of course, it’s on the exhaust side throwing the 02 off...

I saved the freezeframe data of the event:
C286BFEF-ED85-432C-A370-E17243860C5C.jpeg
6877609A-E1CC-4AE2-96C6-732DABB0099E.jpeg
7B0098BC-4AC6-40C6-8A78-F74A944933F5.jpeg
405A587B-561B-494F-A531-18CFBF48DCF4.jpeg
51F0CCCC-0E5A-45D5-AC62-44E2307816A3.jpeg
C8D8D9FF-856C-42EF-85CE-F59A59B82966.jpeg
 
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I just had this code on my car.... The lean code was due to a cracked rubber part leading to the throttle body... A large crack in one of the bellows of that rubber section caused the car to pull in too much air. Replaced that part and it is fine. My fuel trims were way out of whack... Both were negative short were negative -3 or -4 and the long term fuel trims was -12 to -10 going down the road.
 

ltslimjim

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I just had this code on my car.... The lean code was due to a cracked rubber part leading to the throttle body... A large crack in one of the bellows of that rubber section caused the car to pull in too much air. Replaced that part and it is fine. My fuel trims were way out of whack... Both were negative short were negative -3 or -4 and the long term fuel trims was -12 to -10 going down the road.
But if the trims are negative, that’s a rich condition, no?

and, yes, this sort of code is either weak fuel delivery or unmetered air causing an apparent lack of fuel..

I will have to double check for vacuum leaks but again I would think if it was this obvious at 70 MPH then why would a vacuum leak disappear at idle?
 

ltslimjim

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Update: New Denso upstream AFR sensor fixed the problem.

I was able to swap sensors with a relative that has the same vehicle and the problem went away completely on a test drive.

So, in my case the data that shows a clearly rich downstream condition yet there is supposedly a very lean reading upstream then the chances are upstream sensor is bad.

Thanks guys!
 
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Update: New Denso upstream AFR sensor fixed the problem.

I was able to swap sensors with a relative that has the same vehicle and the problem went away completely on a test drive.

I was reading anxiously, hoping you'd get there!

I wonder if the bad upstream sensor was the root cause of everything. That would likely call for rich mixture which would ruin fuel economy and shortly trash the cat converter.
 
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