Test Your Lean Condition P0171 / P0174 Diagnostic Skills! 

You are a national treasure sir. I have been watching you videos for a couple years now and always learn something new. You present data in such a great manner to teach people.

Matt, I want to sincerely thank you for making this video series! I have a 99 Silverado NBS with the 5.3L. I've been trying to troubleshoot the cause of my P0171/P0174 for 2 months (due to lack of time and money). This series really hit every possibility, in the fastest and cost effective way, and you are a true "troubleshooter"... and that comes from a guy who is a troubleshooter on fighter jets. 2 days after watching the entire series and taking lots of notes (like you said, don't just stop when you find 1 possible issue) my truck was fixed! and I don't have any of the cool toys you do. THANKS BUDDY!!!

Hi Matt, a fantastic video, I have liked a lot of the questions during the diagnosis, this time you will overcome. Please, I would like to do more videos like this. Thanks Master, thanks for your time and for sharing your knowledge with us. Thanks Matt.

Great job explaining fuel trim and problems. Thanks!

I just had had a toyota with both banks STFT at 8 and LTFT at 52 and P0171 & P0174. Turned out to be a piece of fuzz wrapped around the MAF wire. Removed the fuzz and a shot of cleaner and instant fix confirmed on scanner graph. I used all of Schrodingers thought processes to get to the MAF. Thanks Mr Schrodinger. Might I add that AZ free scan wanted the guy to buy a $110 O2 sensor before he brought the car to me.

it's 2 am and i can't stop watching this channel, damn you mat, greetings from argentina

You're a great instructor. I got the right answer right off the bat! But, I didn't watch your fuel trim videos multiple times. Repetition is good for old dogs.

I was a GM mechanic for 20 years and had a wonderful time, all i did was short blocks and engine rebuilds, I was the only guy on the line who like doing them and i got extremely good at one point, I was young enough to install manual transmissions without the need for a transjack, just pick it up and slip it in. BUT But but....When i had to learn an all cars later, I was spoiled and had no experience, I got over it, but was never as good with any other cars, Nice video.

most common problems with po171 dirty maf, fuel delivey and vacuum leak. freeze data show the problem is only under load that eliminates vacuum leak. the problem only affects 1 bank which eliminates dirty maf and fuel pump. o2 sensor shows rich at wot and switching normal. I would check fuel injectors on bank 1 with a fuel injector pulse tester and check that each injector delivery the same amount. thanks for your videos I learned so much from them.

thanks for all the great videos and all the different ways to look at them keep up the good work.

I love all of your videos that I've seen, but the graphical representation of the "ricer flyby" made me laugh out loud. Just one more reason I love watching these.

Trust me. This is not wasting my time. Your videos are golden.

That scotty kilmer choice though....... 😂😅😂😅😂

i just found your channel while trying to fix a 5.7 that hesitates. i like your format (instructor giving quizzes) and you are quite entertaining. keep it up! ever spend some time with performance Audi products? be careful, I used to think American muscle knew no equal. rs7 Performance Premier made me reconsider. being a physician you are able to test drive one without being an imposter/ tire kicker. do so. Those Germans are pretty sneaky a 4 door super car, its almost criminal.

Good refresher, thanks Bernie. 🙂

The best educational youtube channel I have found on diagnosis cars! Great channel wowwowowow thankss

Matt. Your a great tech. I enjoy watching you troubleshooting. All your videos are awesome. Don't be a parts changer.

I enjoy the amount of knowledge I obtain from your videos, they've actually helped greatly getting my grandpa's truck running again. Favorite parts thought are you jokes about that Terd with lips scotty kilmore

I'm Hooked! 20 year mechanic here and your great!

Awesome video, always learning something new from you. 👍🏼

Excellent diagnostic information.glad i found your channel.

Another good video you're kind of right about Scotty he's just trying to make a living too and keep the local auto parts stores in business thanks for the education

A long, long time ago...you were my most watched channel and probably one of the first I subscribed to. Then for some reason that only RU-vid can explain, you disappeared from sight...and short term memory. Fast forward to current time...VR6 fuel trim issues. Upon doing some research, came across these amazing videos from years back. Still some of the best out there. Just wanted to convey my appreciation for the help. Hope all is well.

This is a good example of why not never just go off a trouble code and always look at relative live data for other clues.

If I suspect a part is the problem I usually check it with a multimeter where applicable and I clean sensors, check vacuum lines etc. Need to watch this one a few times Matt and I am going back over the fuel trim videos too. Great channel.

your awesome dude. been a mechanic most of my life, you make it fun again. And leave old scotty alone, hes a good informative guy as well. Gene ps my 2001 4.3 jimmys 171 and 174 codes have been driving me nuts. im not a parts changer but i'll figure it out.

Video was very well explained.

Good stuff. Well explained.

Man don't change a thing. Enjoying the channel! I like the to the point teachings and do crack-up often on your sarcasm. (side jokes)

these videos are amazing!

I like your videos a lot, I learned a lot from you, I feel sorry for those stupid comments from ignorant people, what they don't know is that they waste there own time, thanks Matt.

Wow great videos!! Im learning soo much, plus learning how to properly read my code reader... still don't know why all of my banks show a 24- 35% positive lean on all my STFT and LTFT.. but ill keep watching to hopefully find out. Lol

Hahaha.. this my 1st PID to look at.. thanks sir..I have learned a lot from you and all of this make sense now... thanks again

one tech to another, I like your OCD! vacuum! my man.

I watched parts of this video an all of part 5. I know exactly what your problem is! There is a lose nut behind the wheel :)

This is what I’m looking for to be explained the lean and also diagnosing the system

Fantastic Matt you make a difficult subject fun. Ok I'm going out on a limb here taking a chance on getting a punch in the throat , but here it goes. Both O2 went rich on a WOT, so fuel delivery is capable of adding fuel. Live scan data showed adding fuel bank one but you say no drivability noticed. I am leaning towards a fuel delivery problem from injectors on bank one. I would do an injector balance test to see if you have an injector not spraying equally.

So i answered all your questions correctly. Haynes Manual jockeys...? Back in my younger days a friend of mine put petrol in their new diesel car, easy done when you convert to a diesel after many years driving petrol cars. New cars have an Anti-Syphon mechanism down the filler spout so getting out the fuel without taking off the tank was not easy. He paid a garage to have the fuel taken out and a new fuel filter fitted for a rather large cost. A Haynes manual indicated where the fuel tank could be accessed from without removal, this was under the rear seat and could be done without taking out the fuel tank. The large charge from the garage could have been avoided by reading a Haynes manual to locate such a easy simple port hole to remove the tank cap an fuel lines to syphon, clean and do the work yourself. Since this i consider information provided by the Haynes manual very valuable, as for Scotty Kilmer, Y U hating on him? Fuel injection is a wild fairground ride of fuel economy and performance but Carburettors sound and make you feel good. Can you tell, I have a passion for naturally aspirated engines with float bowls...? Nothing beats the sound of a wide open Dellorto/Webber butterfly with trumpets.

I was Wright on both questions so it means I was a good student sir

Thank you. Really thank you.

Injector balAnce test on bank 1..Matt you are natural, great showmanship you should submit some your vids to one of automotive channels get a 30 min segment for engine performance diagnostics..

Dear Sir,I have a BMW E46 316ci .The fuel pressure was damaged ,so low fuel pressure ,so high LTFT (+20,3).After replacement the LTFT is remaining +20,3 but the STFT is -20,3.My question is how long it takes the LTFT to become close to zero in order to drive STFT close to zero? Thanks in advance.

Using the scan tool I would test the fuel trims and O2 sensors at idle by producing a small vacuum leak in say the brake booster line. Determine if everything (Bank1 and Bank2) responds as expected to a small lean condition. Spray some propane into the small leak and see if everything responds as expected. Basically, go to a known "good" condition and perturb it and see if you get the expected result.

What i'd do is, disconnect all four of the fuel injectors electric connectors. (He's furious already) use a paper clip to jumper those connectors to send directly through the pcm with no load, find a soft part of that trans am and set it on fire, then buy you something made in Japan, this concludes my diagnostic suggestions. Lol btw thank you Matthew, for a highly entertaining and super informative channel.

This was an AWESOME video, 🙂, i bet 'Scotty' really watches this channel too 🤥!! i always crank up your video's to 1.7 X above the 'normal' speed, as then it sounds like you're kicking ASS !! Thanks for all that you do, and please keep up the great work , Bless 😇

I have a very odd issue with my 05 Nissan Pathfinder SE off road v6 4L. I had the p0171 code, but for some reason, it only trips at load after warm up. Something seems to click on, and I can watch the live data just start running up to 25% and staying there until it cools down and is restarted hours later, when it goes back to correcting the fuel trims. Also, the bank 2 sensor 1 goes up to .9v and stays there rich on bank two, so I'm not sure why this happens in conjunction with a lean bank one unless there is a manifold leak on the bank 1 side and all that extra fuel is still registered on the bank two (I swapped out this a/f sensor and captured the same exact readings for both sensors, which also means the old factory one I put back in it is probably still good). I've been putting off ripping into the exhaust manifold in fear of stripping something out without having the proper tap and dies to bail me out. I might lube it up and gradually give it a go and at least see if I can find any leaks hooking up my vacume to the tail pipe. Lol jk vacuum. Sorry, I couldn't resist 😹. I just don't understand why it only occurs under load. Could I collapse a hose or something, or when the fan kicks on or some switch activates it disrupts the O2 sensor signal somehow tricking the computer. Idk. Is there a load switch or something that would just all of a sudden cause the one side of the engine to go lean and the other side to be normal.

Amazing You Best teacher 👍

yeah . i was kidding i find what you offer and how to explain is very useful and is not competitive for you . i have learned so much in this short period . thank you so much for this \ i am not very good in writing English . but i speak well / .

public I like this dude ! I would learn a lot....

I like the scope and scanner videos the best

fuel trim at idle, STFT oscillations from -2.3 to + 2.3% and LTFT still at 0%! is this correct as a fuel correction? Thank you

Hi Bro, could you do examples of testing procedures on ASE A1? I subscribe and I always dig all of your content. If you read this question, please let me know! Thanks again for delivering outstanding content!!!

I had an issue recently with my 2002 Holden Commodore which runs the 5.7 litre Chevy LS1 engine where I drove it after that it sat outside for a few days & had a hesitation when cold. After I filled it up with fuel the next day the CEL display came on & the fuel trims Maxxed out at 25% on both banks,for whatever reason,my scan tool didn't read a lean fault code. I tested the fuel pressure which was at 47 psi (it should be at about 60 psi) & the fuel pump was noisy. I decided to replace both the fuel pump & regulator for just under $50 AUD (the whole fuel sender costs about $100 AUD on EBay),the regulator is a Buick park avenue one which I bought for $9.99 AUD,the pressure had to be adjusted in a bucket of fuel whilst running the engine,it was set at 61 psi which is the best I could do. The fuel trims today were at 10.94% on both banks @ 100 km/h (60 mph) at cruise speed with an ambient temperature of 23 degrees Celsius & a coolant temperature of 100 degrees Celsius. Fuel trim under load was at about 7 - 8 %. So I'm glad that problem is all sorted out now.

Hi Smith. Thanks for the good work. Please tell me about the TOAD Master scan.Thanks.

Tnx to be honesr u really are a good teacher

Great videos. I'm here a little late for the quiz. First time learning how to troubleshoot with a scanner and came across these vids. Anyways, greetings from Grand Junction CO.

Regarding further testing to diagnose your P0171 issue, First, I would go to Mode $6 data, in Global OBD II, to see if there are any active misfires to identify the offending cylinder(s). If yes, then I would consider an injector balance test (assuming I had access to an fuel pressure gauge and I could activate individual injectors with a bidirectional scan tool (If no bidirectional capability or support, then use a Fuel Injection Pulse Tester (e.g. OTC 3398)). If no misfires, then I would pursue an exhaust leak. To test for exhaust leaks, obviously one would need to smoke test the exhaust with a smoke machine (e.g. Smoke Pro).

I'm looking at the graph. I truly believe it's the O2 sensor. I clean mine up just a little bit not too long ago. And then on day two it came back up only because I cleaned it, but I didn't use. Gasoline like they say you should use. I use brake cleaner to clean it off. But I will be doing a smoke test real quick. Just curiosity.

Geeat videos just swapped a ls2. Into my Jeep and chasing this exact issue

My best guess is that you may have a loose or leaking exhaust manifold in bank 1. I would take a bottle of propane and introduce propane along the manifold seal while watching the fuel trim for bank 1. Dropping the hammer on rice burners will create a lot of exhaust back pressure. Separately, I enjoyed the quiz format. I had several DTCs Friday including P0153, P0137, P2098, AND P2096. Afterwards I recorded O2 volts, fuel trims, and rpms on Autoenginuity and I have that file. I diagnosed the problem this morning using the recording. It is a parts changer special right up your alley. LOL. Would you like the file for a future video?

Your videos are helping a lot but I’m still having trouble diagnosing my problem. I have a 2001 Ford Ranger V6 with a lean condition only on bank one. When I start the car cold, the LTFT is always at 25% and the STFT is usually also around 25. When I increase RPM to about 2500 RPM the STFT drops usually reaching -15%. LTFT slowly goes down as well. I am pinpointing my issue to a vacuum leak, an intake manifold leak or maybe an exhaust leak. Could anything else cause this issue? Fuel injector maybe (but I would think fuel trim would increase at higher loads). My check engine light used to blink with a misfire on cylinder 2 but, now misfire code does not appear even though I feel it when engine is cold. Any suggestions?

Never mind. You caught it. I bet this would have been fun live!

I like the sarcasm

Hi Matt, Great Stuff! Got to keep doing them like this. The Injector Balance sounds logical for Direction to me because I believe you said you have a Schrader Valve handy on the Fuel Rail to hook up a Pressure Gauge. It make for a quick and easy start in your Direction. But it also raises some questions. If one Injector is somewhat restricted and or partially shorted internally, would you not get a hiccup/dry misfire under load? Also if the Injector was restricted, at WOT why is the Bank 1 O2 going fully rich? If you are saying that the Computer is calling for more and more fuel because it is detecting a lean condition under load and that is why the O2 is going rich. At WOT and the engine is inhaling tons and tons of air, would the Computer/Fuel Trims be able to compensate for a partially dry cylinder, enough to send your Bank 1 O2 fully rich? Just a hunch, could there be a small exhaust leak upstream of your Bank 1 O2, or could the bank 1 O2 be a little lazy? Just a hunch, not a guess, so I dodge the punch in the throat.

whats a good meter to get that does live data?

Vacume 😂. I'll keep it mind!

I have a question do u know what it could be when my ls1 knocks under load right at 1200rpms any less and there is no knock I have lean codes for both bank 1 and 2 cylinder 1 and 2 the engine sounds fine at idle but knocks on the left side of the engine under load

So what kind of glass packs do you have on your Fire Bird. I had a '69 Tornado with duel exhaust and walker continental glass packs that sounded so mellow with that 455 cu. in. high compression engine under the hood. Last year they made that engine in a high compression model. So cool when the smoke would curled around the front fenders. That was a big step up from my Morris Minor.

I have a lean code I’ve been trying to diagnose for a while now. Your videos have been very helpful but I still can’t pinpoint the problem. My fuel trims are fine at idle but but my LTFT jump up to 40% under load. My freeze frame shows usually at 2k rpm. Cleaned my MAF, inspected and changed my fuel filter, cleaned my injectors code still was there. Checked for Vac leaks for the hell of it and nothing. Inspected my TPS and noticed it was out of range so I recalibrated it. Changed the cats because they were 15 years old and the O2 sensors. Changed my injectors along with fuel pressure damper, and my fuel pressure regulator along with my fuel pump because I have a spare. I want to test my throttle body motor and APP sensor because I know they all feed into my fuel trims. Is that a good next step? Any information on how to test these? Any help would be appreciated.

if your there i have good question i think. Have you ever taken an exhaust off to do a transmission perhaps. And when you were all done you had a system too lean. fuel trim suggest its a vacuum leak. how would i eliminate an exhaust leak?

First person to explain it where i actually understood. Thanks

Thank you

Thanks dear

I have a 2002 Olds Silhouette with a P0172 rich code not a mechanic but have a cheap code reader and have a Long term trim -30 and short term is around -0.8 what do you think the issue is?

What would be interesting is a person with a common problem and the ability to provide graphs as needed to provide you with ongoing requirements as required by yourself and then attempt to diagnose the problem.

2007 GMC Yukon 5.3L Engine: I get a P0171 code when running the engine with the AC on for about 20-30 minutes. Vehicle begins to jerk and shake on idle. After the vehicle cools of or I turn off the AC, it eventually returns to normal function. Any ideas?

OK, a car running lean on one bank. We can eliminate MAF, MAP, O2B1 because of .1 -- .9 swing, I think that leaves us with bad fuel injector. A power balance test is in order?

Was there ever any solution to the corolla problem.i have a 04 corolla with same code .engine runs fine only a bit of rough idle when parked.

I got it right! So in tradition on this channel I will gloat and be full of myself!

i had a 99 t/a it ran great for 10 yrs when i had it..

Matt. I just wanted to let you know that during the valve adjustment I performed myself on my 2006 Acura TL, I used your method of turning the crankshaft with an over-set torque wrench as a massive ratcheting breaker bar from the land of intelligence. I have multiple misfire codes during WOT. (P0300, P0301, P0302, P0304, recently P0306, all usually pending unless I keep the WOT going despite the flashing CEL.) it appears the misfires begin at 3,000rpm, but below that, there is no misfire count. I recently installed an OE fuel pump module, as while chasing this misfire condition for over a year prior to finding your channel, I had installed a Delphi fuel pump module in February as part of a near weekly parts cannon entry because I found it for cheap and figured the in tank fuel filter neede-… you know already. It turned out to cause a starting condition, P0171, 01714, and driveability issues that were rectified by the genuine fuel pump module, whereas the Delphi pump did not originally fix my misfires. I am now suspicious of my injectors, but this unforgiving, returnless system with integrated pump, filter, and pressure regulator, and no way to test fuel pressure under load to my knowledge due to needing to remove the intake manifold to test fuel pressure at the rail, I’m feeling stumped. I lack scopes.

Great Info Vid 👍🏽Love Going to RU-vid Mechanic School lol U the Man Teach 💯💯💯

Is there any way a tps sensor (located on the fuel intake.) could be a part of all this?

I have lean codes on both banks and as I accelerate my short term fuel trims rise from 0 to more than 20% and my long term fuel trims lower from around 26 to 10. Could this mean a vacuum leak?

I’ve been dealing with a lean condition b1&2 on a 05 gx470. It’s acts like a vacuum leak as the Ltft’s calm down to below 5 under hard acceleration. Mid 30s cruising 35mph under vacuum. O2’s appear to be reading and reacting correctly. I’ve gassed around the intake and haven’t had any fuel trim reactions to propane. I have not got the fittings to do a fuel pressure test yet. It runs good under hard acceleration, but surges above 60 under light acceleration. Has anyone seen the sais check valves not close fully and allow air into the exhaust without codes?

can you how to show testing for evap leak ?

I have been watching many of the videos and I can not seem to find a way to differentiate a bad maf sensor from injector imbalance. I have a corolla 2005 that I am getting a p0171 code in the winter. At idle I get 7 ltft. When I accelerate it jumps to 15 ltft. Checked the fuel pressure and I got 45 psi, which is within spect. what test can I make to see if its maf or injector balance.

If you’d lived close to me I’d give u two of my checks to diagnose my cars lolllll Your a Diamond mechanic

I'm actually very excited to find out the cause because I have similar problem with my Corolla :D. LTFT goes high under load. I have of course looked for simple and unlikely stuff like spark plugs, vacuum leaks and other leaks and did O2 response tests (was switching fine and did respond to lean and rich condition). Then I had my fuel pressure checked which was perfectly within specs. Then I took my injectors out and cleaned them (not much dirt came out) and spray pattern looked fine to me in all of them, so no much effect. I came to conclusion that my MAF was going bad and replaced it (of course I tried to clean it before). And it had a BIG effect on LTFT, dropped from +10 to 4 at idle and from +20 to 12 under load. But the characteristic is still there, so what the heck could still be the problem? Of course there was no way for me to measure fuel volume of injectors so they could still be potentially the issue. I also don't have any drivability issues, just a little higher fuel consumption. And as for this Trans am case, I think the problem is with injector(s), maybe clogged/dirty.

The Chuck Norris reference. Lol

Haha I think I enjoyed this video the most of all of yours so far. I was gonna crack a lame joke about vacuum, but all the ones I can think of suck... From FASTTEC, I want to eliminate fuel, or lack thereof, next. It looks the most promising line of inquiry. I dont think lift pump, im not sure if theres 1 high pressure pump or 1 per bank, if the latter i want to test it. I expect there is only one high pressure pump so moving on, I want to know the injectors are firing. I would use an inductive pickup on my scope (through an attenuator because my scope is only class 1 safe) and I'd hold it to each injector in turn. There is no need to rev the engine or put it under load, you can diagnose if the solenoid is actuating from the trace at idle. Lower tech I would use a stethoscope, I want a nice clean rapid clicking sound from each injector.

Doing a injector balance test is defiantly the next test you should do. But I am going to go out on a limb and say that will not be your problem ! I think you are going to find a air leak in exhaust in bank one. And the o2 is working the way it should.

Is it possible in a map system to have a vacuum leak but not raised rpm at idle?

Hey @Schrodingers Box I have a terrible experience. I drive 07 Honda CRV 2.0l (Stick/Manual) than lacks power. When I floor it on the road, the car doesn't react to it. But when you pull your leg off the pedal, just before your leg is off the pedal power kicks in. So by deduction, when you lightly touch the pedal, power response. I took the car to a dealer for scanning, first I got a code that AF sensor failure, changed that. They later suspected PCV Valve that was ok. No more codes. Injectors work fine. Fuel pressure ok. Where can I go to look next?

i have a 2001 Chevy Cavalier with a code PO171, how can i fix that ?

I guess I passed. I only had 1 incorrect answer, question 3. On to part 2 of the test.

Hmm my first indication was a maf but after researching the car its not a dual intake like the Euro's twin banks I work on so I am inclined to suspect a faulty or contaminated injector....02 readings show the sensor is operational and fuel pump fault would affect both banks....being a single maf it would show a 2 bank fault if the fault was there. Obviously not a vacuum leak. Balance test? Or could we devise something fun?

My LTFT is like around -8 and -15. Does it mean the condition is rich? What can cause it?

Good video. I'm thinking fuel delivery problem(fuel injector/s) on bank 1. Since LTFT is a learned value you will not see the O2 drop lean. The O2 seems to react pretty normal at idle so I dont see a problem there.. A injector leak down/flow test or swap injectors, depending on whats easiest.

if the 02 sensors are reading rich which means theres too much fuel it excludes bad fuel pump/low fuel pressure. but if the maf sensor is bad it can trick the fuel system into thinking theres too much air which will cause a rich mixture and both o2 sensors stuck on a rich condition. does that sound correct?