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I've been struggling with a p0171 code since I've bought my car (second hand). It's a Honda hrv 2016. I've already taken to Honda service several times. They made me change the lambda sensor, maf sensor, fuel filter and still no luck. On long drives (+400km) it's turning on this code (P0171)

I was able to make readings with torque app while I was driving on this code. The 02S1eq values that normally should show "1" , are showing "2" (picture attached)

Can you guys give me some kind of direction of where to keep looking to solve this?

Thanks in advanceenter image description here

I've also uploaded a logfile captured with Torque of a short highway drive forcing this behaviour (above 75/80 mph) Link below https://drive.google.com/file/d/1cGT0b9kCjd8hxB42L6vUo1pgk6Mi8WYe/view?usp=sharing

Thanks all for your help!

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  • Welcome to Motor Vehicle Maintenance & Repair!
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jan 2, 2020 at 2:12
  • The first thing to check with this code would be a vacuum leak. Check for anywhere air can be wrongfully introduced beginning at air filter to valves. Bring it to a different independent shop. It is best to get some references. This shouldn't have gotten this far without a correct diagnosis.
    – Jupiter
    Commented Jan 2, 2020 at 14:12
  • Thanks Jupiter! As I wrote above it only happens after driving on highway more than 300/400km nonstop. Rest of the time it works ok (city drive, short drives). Should this behaviour help to reduce possible issues?
    – Guz
    Commented Jan 3, 2020 at 2:09
  • I'm surely be sending it to a specialist, because they've already made me spend a lot of money with no results. That's why I've decided to study this a little bit on my own. Problem is I'm not mechanic, just a computer analyst. At least I can collect and analyze data :)
    – Guz
    Commented Jan 3, 2020 at 2:12
  • Your short-term fuel trim (STFT) is WAY high, and your long-term trim (LTFT) isn't particularly awesome, either. That makes sense in context of a P0171, though. I agree that a vacuum leak is likely. Other options would be a failing fuel pump or bad O2 sensors.
    – 3Dave
    Commented Jan 3, 2020 at 16:22

1 Answer 1

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P0171 is a "System too lean" error code. This is caused by the engine receiving more air than fuel so it compensates by adding more fuel (hence positive LTFT and STFT). A common way to diagnose problems with P0171 is by checking short term fuel trim (STFT) at idle and at around 2500 RPM. If the STFT is a positive value at idle and goes back down towards 0 at 2500 RPM it indicates a vacuum leak somewhere between the MAF and cylinders (due to high RPM creating a strong vacuum making any leaks insignificant). If the STFT stays the same at both RPMs then the problem is likely an issue with the fuel delivery system, not a vacuum leak. I believe this is your case as you mentioned these values at highway speeds. Since your fuel filter has been changed I would recommend also checking the fuel injectors as they might be starting to get clogged.

EDIT: I just took a look at your excel sheet you posted in the comments. I noticed that as your RPM goes up towards 2000 the STFT goes from 46 to 0. This is a definite sign of a vacuum leak as mentioned above. There might be a little hole anywhere after the MAF. You should look for tears or fractures on any intake component leading to the cylinders. A small hose may have came loose or has deteriorated due to heat and is leaking. I recommend searching for techniques for findings leaks as finding it can be quite challenging. Otherwise it is quite safe to drive as the issue disappears during normal driving (only occurs at idle), only your fuel consumption might suffer slightly.

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  • @Guz I've taken a look at the spreadsheet you posted in the comments. The STFT drops from 40 to 0 as you go up to 2000 RPM, a typical sign of a vacuum leak, I've made an edit to my answer, please take a look for more info. Thanks.
    – TheSlavMan
    Commented Jan 15, 2020 at 18:53
  • Just saw your edit. Those lines showing 46 stft are at the exact moment issue appears. If i slow down, it immediately restore to "normal" values. It looks like sensor gets offff. Lambda values get stucked at 2, stft get stucked at 46, and o2 voltage stucked at 0.9v. Slowing down speed immediately goes to normal values. =(
    – Guz
    Commented Jan 15, 2020 at 18:59
  • I was wondering what you mean by normal values? Your other post the FTST stays between -10 and 10 which is not a sign for concern (O2 seems ok too, maybe a slight glitch)? If you leave your engine running at idle (for 1 min) what is your short term fuel trim? Then if you run your engine at 2500-3500+ RPM (for 1 min) what is your short term fuel trim?
    – TheSlavMan
    Commented Jan 15, 2020 at 19:11
  • By normal values I mean, Lambda ratio = 1, STFT < 10. In the other post all data was recorded with car stopped (where everything seems fine). In this post values are taken on the run, where I start to see this malfunction. Weird thing it only goes crazy at high speeds (75mph). It's really complicated to diagnose something that's only happening at certain not so easy to replicate on lab
    – Guz
    Commented Jan 15, 2020 at 19:58
  • @Guz Yeah it seems in your other post it's running rich with O2 staying above 0.5 for most of the time, it should fluctuate from 0.1 lean to 0.9 rich if you feather the throttle but even when you change RPM it stays mostly the same. I agree that it might be O2 being faulty causing the other STFT issues. Has it been replaced? also if grease/oil/silicone gets on the outside part of the sensor it can give bad readings.
    – TheSlavMan
    Commented Jan 15, 2020 at 20:18

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