P0171 code, but negative LTFT (and STFT)

Hey guys,

I have a 2005 Peugeot (it’s French for “regret”!) 407 V6 3.0L Petrol that I purchased for cheap and have had to carry out an extensive list of repairs from a lack of maintenance on the vehicle.

I am getting a P0171 lean code, but fuel trim remains negative ( from 0 to -5%). Vehicle runs a bit rough on idle and considerably more so under load.

It isn’t fitted with a MAF (just an IAT between air box and throttle body), but I have checked in many other areas (vacuum leaks, PCV, replaced air filter, cleaned injectors, checked injector wiring, replaced plugs, checked coils).

If the O2 sensor was reading lean, then I would expect to see positive fuel trim numbers.

Any clues would be much appreciated, thank you.


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lc1927 wrote: Hey guys,

I have a 2005 Peugeot (it’s French for “regret”!) 407 V6 3.0L Petrol that I purchased for cheap and have had to carry out an extensive list of repairs from a lack of maintenance on the vehicle.

I am getting a P0171 lean code, but fuel trim remains negative ( from 0 to -5%). Vehicle runs a bit rough on idle and considerably more so under load.

It isn’t fitted with a MAF (just an IAT between air box and throttle body), but I have checked in many other areas (vacuum leaks, PCV, replaced air filter, cleaned injectors, checked injector wiring, replaced plugs, checked coils).

If the O2 sensor was reading lean, then I would expect to see positive fuel trim numbers.

Any clues would be much appreciated, thank you.


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Sorry- I should have also mentioned that my scan tool offers freeze frame, but isn’t letting me view it on this vehicle and this is the only code (started with 13 codes and down to 1!).


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Hello and congrats on your new purchase . I assume the negative 5% fuel trim is taken at idle? What does it do at higher rpms say 2500? 5% either positive or negative is within spec some vehicles differ slightly but I use 10% as a rule of thumb as acceptable. My bet is your throwing this code at cruising speed and not at idle to bad we can't see the freeze frame to know for sure but checking fuel trims at higher rpms is a quick test (do this check both cold and hot engine)

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I agree that there must be a specific condition when the P0171 was set since the current fuel trims are within reasonable limits. If you cannot reproduce lean fuel trims it would be best to seek a scan tool that will allow you to look at the freeze data to get a better idea of the conditions needed to reproduce the P0171.

Ben wrote: Hello and congrats on your new purchase . I assume the negative 5% fuel trim is taken at idle? What does it do at higher rpms say 2500? 5% either positive or negative is within spec some vehicles differ slightly but I use 10% as a rule of thumb as acceptable. My bet is your throwing this code at cruising speed and not at idle to bad we can't see the freeze frame to know for sure but checking fuel trims at higher rpms is a quick test (do this check both cold and hot engine)

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Hi there Ben, thanks for the reply, much appreciated.

The fuel trim numbers are based on a normal operating temp (90C/190F) at idle and I did check at higher rpm. I will perform again and make sure I hold at around 2500 for the sake of accuracy though, good suggestion- also will monitor more closely from cold.
I actually can’t yet monitor under load driving it, as I still have a couple of things to do on the vehicle before I can get it registered for road use unfortunately.
Agree that the freeze frame would be nice..

I will update once I have checked the numbers again with those variations, but failing that I am a bit stuck on next direction.

I never get a code for bank 2, so I am really down to fuel and ignition on bank 1, vacuum leak on bank 1 intake side or faulty O2?


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EricGoodrich wrote: I agree that there must be a specific condition when the P0171 was set since the current fuel trims are within reasonable limits. If you cannot reproduce lean fuel trims it would be best to seek a scan tool that will allow you to look at the freeze data to get a better idea of the conditions needed to reproduce the P0171.

Thanks Eric, I think you are right there- plus I could then see all the available PID’s, where some additional info might become helpful.


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Hi guys,
So I monitored data from cold to op temp and nothing significant, but under increased RPM, found something interesting.

LTFT is fairly consistent over bank 1 &2, but at 3250 RPM (my foot got a bit carried away, started at 2500), I see STFT11 is lower than the others. I known LTFT is the one to be most concerned with, but could this provide a clue? LTFT is still showing lean throughout the rev range. (Images attached)




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Feel free to disregard those ST FTRM11% and ST FTRM21% PIDs. They're not true fuel trim readings, just misleading PIDs from when OBDII was first developed. The ST FTRM1% and ST FTRM2% are the ones to watch.

Long term trims of -4.7% and -3.1% is pretty excellent overall, so I wouldn't read too deep into that. I agree with Eric - without driving it, you may not be able to reproduce the conditions under which the P0171 set.

I agree those fuel trims seem well within acceptable limits. There are so many dynamics within an automobile that seeing a slight shift in fuel trims at one point isn't automatically a sign of what's happening at a different rpm, load, gear etc. The threshold for a P0171 or P0172 is pretty significant.

Tyler wrote: Feel free to disregard those ST FTRM11% and ST FTRM21% PIDs. They're not true fuel trim readings, just misleading PIDs from when OBDII was first developed. The ST FTRM1% and ST FTRM2% are the ones to watch.

Long term trims of -4.7% and -3.1% is pretty excellent overall, so I wouldn't read too deep into that. I agree with Eric - without driving it, you may not be able to reproduce the conditions under which the P0171 set.

Hi Tyler,

Thanks for the reply.

I’m safe to ignore that figure, even when 2 & 21 match up? I wasn’t aware that those numbers were redundant, so appreciate you letting me know that.

I did clear the P0171 several times and it was only thrown at idle and from moving the car within the yard at home- unfortunately without freeze frame I can’t determine supporting figures from when it appeared. The fuel trim numbers I have seen throughout don’t support the fact it is thrown- confusing!

If I ignore the code for now until I can register it and drive it, my concern is that it wouldn’t pass a roadworthy inspection with the code present (especially if they check the O2 with a sniffer and potentially see it running lean.. which it may not even be, though..trim number suggest that it isn’t lean at all).

I still have rough idle and under load in gear, so I would be happy just treating that condition and ignoring the P0171 for now though.

I still suspect a vacuum leak/PCV issue on that front, if I ignore the trim numbers.

I definitely agree that the trim numbers are fine from the scan tool readings provided.



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Hi Eric,
I agree- there is nothing in those figures (throughout various RPM) to support the P0171 code- if I was seeing larger +\- figures, then I would totally understand.
Rather strange indeed. Thanks so much for your comments.


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UPDATE

Found a crack in PCV hose.
Vehicle uses corrugated plastic hoses and these have gone hard and brittle over time. When I removed the PCV valve, the cracked hose split in 2.

Hopefully once the this is replaced my idle issue will be resolved, lean condition resolved and hopefully spongy brake pedal resolved- which I thought was narrowed down to the master cylinder, but most likely the air leak that would have upset the booster vacuum.

Will report back to confirm, but hopefully this was all it was, which is a fairly easy fix.


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lc1927 wrote: I’m safe to ignore that figure, even when 2 & 21 match up? I wasn’t aware that those numbers were redundant, so appreciate you letting me know that.

Yeah, those PIDs are weird. :silly: Most of the time, they're pretty reasonable, and close to what the actual fuel trims are reading. I suggest ignoring them because at certain times, especially during load changes, they can be a bit misleading. Here's an example:



Short term on bank 1 is showing -5.4%, while short term B1S1% is showing -12.5%. :blink: 5% is no big deal, but 12% could be cause for concern.

I did clear the P0171 several times and it was only thrown at idle and from moving the car within the yard at home- unfortunately without freeze frame I can’t determine supporting figures from when it appeared. The fuel trim numbers I have seen throughout don’t support the fact it is thrown- confusing!

I'm not certain about Renault, but I have run into Volkswagens with lean codes that show fantastic fuel trims in OBDII. I go into the learned block values on the OEM side, and bam, the trims are all screwed up. Point is, it's possible that OBDII isn't telling you the truth about the trims.

Found a crack in PCV hose.
Vehicle uses corrugated plastic hoses and these have gone hard and brittle over time. When I removed the PCV valve, the cracked hose split in 2.

Hopefully once the this is replaced my idle issue will be resolved, lean condition resolved and hopefully spongy brake pedal resolved- which I thought was narrowed down to the master cylinder, but most likely the air leak that would have upset the booster vacuum.

Nice! For science, let us know what your scanner says about the trims after the fix?

Thanks Tyler, I can see your point with those weird numbers there too- I no longer feel alone in a world full of numbers that make sense!

European cars certainly have some quirks about them at times (understatement)

In the name of science, I will definitely be back with another update.

Appreciate the help from everyone here to date, cheers all


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Hey all,

Just updating, fuel trim all good and no more lean code (phew!).

Maybe it was spark plug replacement, maybe it was O2 sensor clean, not sure.. but that is resolved, it seems.

Still runs a little rough (though, it wasn’t driven for 4-5 months when I got it and service history info is scant, so really need to drive WOT a bit to get the rest of the injector clean through it and see if that helps any). Soaked the injector nozzles and there was quite a bit of carbon. Budget doesn’t allow for pro clean or replacement at this time, as I am getting this car on the road for my girlfriend and she is a bit poor at the moment (“what, I have to get it serviced every 10k kms?...why??”.. “it needs more parts?!..ugh..”).

Unfortunately the vacuum leak fix (temporary piping in place- as a replacement PCV valve & attached hoses has to come from UK.. and I am impatient) didn’t help with the soft brake pedal like I hoped it might. So a new master cylinder is on the cards, as there must be air entering there (no ABS faults, lines bled and booster seems fine- no leaks from master cylinder, but the pedal does continue to sag when engine is running).

Fun times!

Thanks for the help guys.


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Air in the system can cause a low or soft pedal. Also, stuck brake pads can cause a low/soft pedal. So can misadjusted rear brake shoes.

I worked on a Volvo S80 once that had a really low pedal. The parking brake went all the way up without engaging as well. I found both rear brake caliper piston mechanisms ( integrated parking brake ) defective. The Pistons would move out as you pushed the pedal but would retract almost completely into the caliper afterwards. New callipers fixed that one.

If a vacuum leak was bad enough to affect the booster, it would be unlikely to manifest in a brake pedal fade- your pedal would be more stiff and require more effort.

If the vehicle sat for 5 months, look at the rotors- sometimes they get rusty and they don't feel good until it all gets scraped off. (if its safe to drive) I would get them on a couple 60-0 mph stops to get them warm to see how they feel. If no luck, try bleeding the brakes thoroughly- I would certainly go after that before spending money on a master cyl..

Good luck!

Thanks for the reply-
Agree on air in system as the most likely, but have bled at the calipers (in the correct order and did a second time, just to be sure). So, beyond this I gather that air is being dragged in at the MC perhaps. Fittings all ok and no line leaks anywhere (no leaks from MC either).

Interesting thought- the caliper pistons were not ceased, but potentially something there, will inspect the calipers more closely and make sure the pistons are retracting correctly etc.

Park brake has good engagement and 6 clicks on ratchet to fully on.


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Another possibility for a low brake pedal is if the brake pads are stuck in the bracket the piston will flex the pad till it contacts the rotors then when pedal is released it pushes the piston back into caliper so you won't see an air gap between the piston and the pad but you will between the rotor and the pad I usually only see this on vehicle with 4 piston calipers.

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I agree- just wishful thinking that a minor leak may somehow be related.

Definitely a bit more rust there than you would find on a car driven frequently, but that will come off when I can drive the car on the road and would certainly affect the braking performance, but not my pedal feeling whilst stationary.

Already bled twice (second time was just to convince myself that I did actually do it, as I thought surely that was the problem). Will take another look at it today.

Thanks for the input


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