P0171 (lean), possible false reading
 

So i originally posted to Facebook, but i know the forums are a bit smarter. That and this information may help someone else. Anyway, what i got:

Need some help with a P0171 / lean code.

2013 with 190k miles. NA, decent amount of mods, but nothing new in the past few years, so issue is not mod related. Had this thing since new.

Rough idle, drives relatively fine otherwise. LTFT is near 40 at idle. Abnormal smell of fuel at idle, seems it's adding fuel that's not needed, leading me to think it's falsely reading lean and trying to correct. Fuel system seems fine, it's dumping extra fuel at idle and able to handle higher RPM fine, more likely exhaust / vacuum related. Checked vacuum lines, including that sneaky one under the intake manifold, all seem good. Exhaust has no leaks / cracks. MAF sensor clean, figured i'd check intake filter while i'm at it though likely not related, also clean. Cables are good for MAF sensor and upstream O2 sensor, no signs of damage.

Only two things i can think of are either bad upstream O2 sensor or maybe bad fuel. I did get fuel from a gas station i don't usually go to, a 7/11, before the issue began to arise, hard to believe the one time i did get fuel there it would be bad, and it's 7/11, not some super sketchy place that would buy some black market fuel or whatever. Upstream O2 has been there since new.

Only past issue i had before was a single coilpack going bad, front passenger side, likely due to added heat from modified exhaust, that was about 6 or 7 years ago.

I have a new upstream O2 sensor on the way along with a new MAF sensor because why not. Was going to drive / burn as much of this tank as possible and refuel at a more reputable gas station to check off that potential cause. Anyone have any other ideas / possibilities?

UPDATE: fuel from Shell seems to run much better, but still high LTFT. Moral of that story: fudge (this particular) 7/11. Logs show O2S2 reading 0 volts, mixed messages in forums about it always reading 0, but i'll zero out A/F #3 in Romraider to see if that helps. So maybe bad downstream O2, though no idea why it'd affect fueling as it's primary purpose is to monitor cats.

Are you able to post a log? That may help. LTFT of 40 is pretty extreme. The fact its at idle mostly has me leaning (lol) towards a leak of some kind rather than sensor failure. What mods do you have?

A handful, but nothing recent, last possibly relatable mod was installed several years ago, i couldn't find any leaks anywhere. Issue came up over the course of a day, after filling up at a 7/11, but i think the 7/11 part is just coincidence. Mods that could be related:

Modified intake snorkel, filter, tube, intake manifold spacers, headers, over pipe, front pipe

Will also note i cleaned MAF and MAP sensor too, no luck.

I can post a log, but no idea what parameters should be logged in particular to better help

What was your method of leak detection? Visual is usually not good enough. A smoke machine and/or spraying carb cleaner on suspect areas is best.

I’m interested to hear your solution.
Our 2007 Yaris (290,000 kl.) throws the same 0171 code periodically. When it does it is always at the start of a highway journey shortly after getting up to speed.
The car runs flawlessly at all times. Google says it could be MAF sensor, PCV or #1 O2 sensor. I have cleaned the MAF but the code still pops up occasionally. Sometimes it self clears while other times I clear it with my scan tool. The O2 sensor was replaced with an OEM unit last year after a muffler shop damaged the original sensor somehow. It has never thrown the code during city driving. I will check the original PCV hose and PCV for issues. The PCV is several years young. I cleaned the original one after switching it out. Perhaps I’ll find it and try it again, if the hose is okay.
Thankfully my CSG tuned FRS runs strong and flawlessly.

Visual, sound, and feel. Didn't find anything. Though not needed, decided to ziptie some vacuum lines down just in case, no improvement. Will try to find soapy water and spray bottle

Also, checked AFRs and they're near command if that matters. Zeroed A/F #3 corrections too, no improvement.

Correction is done off the O2's. There is no way you can be smelling fuel and have AFR's near command. Do you have anyway to read sensors from OBD2? Torque app or an OFT?

I use OFT, but not 100% sure what parameters to read. I've primarily been watching MAF voltage, command AFR, AFR, STFT, LTFT, and a few other things.

The weird readings is why i'm thinking faulty O2, but if it's faulty then how's it getting to command? Of course that may be part of the faulty issue i guess?

This is why i'm here. I have a general idea, but no idea what i should actually be doing.

Log the following parameters.

Fuel system status ol/cl
AFR (actual measured by O2 sensor
Commanded AFR ( from OL Fueling table data)
LTFT
STFT
LOAD
MAF Volts
Throttle position
IAT
RPM

Will do, won't have any logs until Friday/Saturday though. Should have new MAF and upstream O2 installed by then too.

02 Sensors tend to fail in a lean condition direction. So if the 02 is going bad, it is telling the ecu it is leaner then reality, and the ecu then provides more fuel. This would match your symptoms (ECU believes it is at correct AFR, but it is in fact far richer).

Yes, this makes more sense. So seeing O2 sensor voltage would help. We can compare to existing 02 scalings to see if it is in range. It has been a long and hot year :bonk:

No...the voltage will correspond with the AFR reading the ECU is seeing.

The only way to test would be to swap in a known good 02 sensor.

Yeah, you are right. I must be slipping, twice in one thread :slap: