Fuel Trims - Where to Begin your Diagnosis For Rich or Lean (p0171, P0172, P0174, P0175) 

P0171 basically means you have a vacuum leak usually the first place I go to the gas cap to make sure it was tightened properly because if it is not then you're going to show a vacuum leak.

Thank you so much for taking the time to explain everything!!

Gracias maestro. Veo sus videos en español pero en inglés un poco. Gracias nuevamente por su aporte a la enseñanza

Blackbox is ecu. New stuff to learn for old timer.I ask my kids if I have trouble with my pc. Lol. I wondered how my bike starts without choke, now I know.I have to watch more of your vids. Great stuff.

South Main Auto sent me to see your class, Great program, I need lots of this and will follow more videos.

Thanks for detailed explanations. I appreciate all work you put into making this video.

hi from vegas really love the way you explain things on the fuel trim made me look at diag from a whole different view thanks

Taken on a 175km trip now all looks good (touch wood) STFT around -1.6 Banks 1 and 2 LTFT around -7 Banks 1 and 2 and motor seems to be running smoother. Next I will check Fuel consumption Thanks for your help. Over here in Australia most of the mechanics I have spoken to about this problem just tell me to just reset the codes, this has made my head hurt for o many years, my fingers are crossed. Graeme in OZ

Very informed video, a lot of doubts have been cleared. Your explanation are terrific.

Absolutely Brilliant!!! Thanks for the Detailed Explanation!

thank you for this video , excellent information . I've been wrenching since the 80's and I still love to learn about how cars work and this is a fascinating time to be a tech with all the changes happening year to year . I also didn't see the need for parts to be shown .

My 2001 vw beetle glx 1.8t..had PO172.. Cleaned the airflow sensor and throttle body..Runs like brand new now..As a mechanic for 32 years in Audi Bmw Mercedes I recommend cleaning the airflow sensor every oil change..at 2000 miles..Thank You and I approve this message...

Great presentation Mike and looking forward to your lecture on the wide band sensors. Have a good weekend!

This dude really knows what he's doing!

you are a very good teacher the best on the internet by far the way you explaine things im going to start to watch a lot of your content excellent

Wonderful class...excellent information. Greatly appreciated ..... Thanks and take care

Great channel to keep learning. Thank you very much guys.

Oh my, a God sent gem thank you guys.

Very good video with a lot of helpful information . Thank you. Quick question, My car recently was running lean 21 percent lean long trim and 18 short trim on bank 2 due to a exhaust leak between the 2 flanges right before the cat. I got that fixed and now the car sometimes runs -18 but mostly stays on -6 to -12 range on short trim and bounces from -10 to 3 on long trim. How many driving cycles does it take for it to adjust to normal .So far I drove the car 3 times

Very good, thanks for explaining the details - your words make great sense

A little late to the game Mike, but THANK YOU! for the invaluable content. These classes are pure gold when it comes to understanding how these systems work. This is one of those principle concepts that if you can understand you can troubleshoot a lot of issues that we face as techs. I found your channel from Eric over at SMA and I'm sure glad I did. Cheers from Stevens Point.

Okay after watching the this video training my question was answered! Great training thanks!

I'm a new subscriber love what you are doing. Hopefully I can catch up and earn a shirt and thanks for taking the time to share great info.

Hello I hope you're doing well just would like to have an idea why the hose that goes from the canister purge valve in the Gasquet it would shrink when accelerating i would really appreciate it thanks

Mike- we just had a uplander in class with rich codes and intermittent hard start. FFV, alcohol content was reading 75% but actual content in fuel was less then 10%!

Such a great teacher...god bless you

I have a p0172 code, running rich. Only went on when there was a cold snap and engine light went off when weather warmed up. By cold I mean -30c then warming to -6c. Could a sensor freeze?

Better than tele. Keep up the good work. Stft and ltft no longer a mystery. Thanks.

I have 2004 mini cooper 1.6l r50 check engine light on and code p2097 post catalyst fuel trim system too rich bank1 what should I do? somebody said I should change a oxygen sensor... but which one should I one on the hood or one under bottom of the car?

Also for the vehicle to be running rich, you could be getting blow by and the air gathering up oil and taking it through a failed crank case vent or pcv into the intake whereby it would be burnt and making the exhaust rich.

thanks so very much for this wonderful technical talk

Thank u for educating us, I always enjoyed your videos

Question: I have a 2013 F30 BMW 328i. I got codes P0171 and P0172 at the same time. The codes contradict each other. My "start/stop" feature in the car doesn't work anymore. Any idea what the issue is?

I enjoy watching !! thanks 👍🏼

At 33.12: it's not 260 seconds but 260 frames on the display of the snap on scanner. Thanks for your time and effort.

Had an envoy with a p0171 an a p0496 p0300 with long term at 35+. missing an intake vac line cap an purge valve was stuck open still checked the maf.

Very useful information sir thank you Sir I have Chevy suburban 2007 and I'm getting codes p0172 and p0175 system too rich on both banks and ratio is showing short fuel trim at 0 and long fuel trim at -25 what do you suggest me to do first

I just found your site. I am new to this and it is very interesting subject. i just bought a new Altel 808 and it amazing to watch it on live data. I have a 1999 gmc k2500 5.7 I have replaced the injection spider and cleaned the carbon out of the intake. I Have changed the o2 sensors and it is telling me that i have a low heater out put on bank 1 sensor bank 2. I have looked at the wiring and it looks good. any ideas. The A/F is 14.7. o2;s are reading 450. The engine is high mileage, does not use any oil, the compression is 160 to 165. starts good and runs great. Thanks

Thank you very much for the information brother. great job.

I had P0172 and P0175 codes on my 2013 GMC Sierra 2500hd. Once I pulled everything up on the computer, I noticed my fuel alcohol content was @ 64%. I reset that and got rid of my rich codes. Now I have a P0300 code and O2 sensor codes for both upstream sensors. Fuel trims are perfect. What could be my issue? I'm planning to check for fouled plugs and maybe bad coils but I thought it was weird I didn't have any misfires codes until after I reset the FAC and trim levels back to zero. Also all 4 O2 sensors are brand new and operate in range.

I got a question so I have a 2003 Ford Mustang 3.8 v6 and my LTFT B1 and B2 are both at -99.5 but no codes at all and engine seems to be running fine what’s going on with it?

My car is a 02 chevy prizim. I keep getting a po 420 cat system low efficiency code. I have already replaced the following. The upper and lower purge solenoid was replaced, Replaced the charcoal canister, changed the up and down stream 02 sensor. I replaced the mass air flow sensor. I replaced the cat. I also checked all the evap hoses and their is no leaks. I also changed all the spark plugs and pick up coils. I'm at my witts end and trying to figure out. Why it keeps giving this code. And keep blowing the cat, purge solenoids and pick up coil. I also did a smoke test and it well sealed.

Wouldn't a vacuum leak affect both banks on a two bank engine excluding a Manifold leak on on one bank? Same on a bad injection or bad plug or coil pack on that bank?

I have a car that can run on petrol and on LPG. When run on petrol I get HO2S 0.3V, stuck lean. When run on LPG, I get HO2S 0.7V stuck rich. Also HO2S is brand new and OEM. Using OBD reader,HO2S voltage is constant, it is not moving from rich to lean. Any idea what to look for?

Hey Mike is good teaching me alot and it my first time watching your video.. but I can't wait to see electronics problem. Cause im not good a lil bit electronic repair or something

Wisconsin!!!! ...and thank you for actually being well educated. So much bad info out there. ...people who think that O2 sensors measure fuel and not oxygen. The moment you said that a rich mixture could cause a misfire, which would actually create a lean reading from the O2 sensor, I knew that you actually knew what you were talking about.

awesome video, great presentation!

I have a Lexus is350 showing p2238, I have replaced the O2 sensor the dtc does not go away. After checking the fuel trims I have 0 on both on short fuel trim but on the long fuel trims they are -3 and -6, any idea about what is causing it?

Do you have any videos that go over closed loop fault? I have a 03 Ford expedition that is stuck in cl fault. I don't know how to approach this issue

I have a p2098 bank 2 code on a Dodge Challenger 3.6L it’s running a lean fuel trim. What could my issue be? I’ve changed all O2 sensors and catalytic converters and still jerks a bit and had a check engine light. I added a fuel injector cleaner LUCAS and it removed my check engine light for 40 miles then it came back???

Once again excelent teaching congrats.

My car is LTFT -10.3 ? but everything like 02 doing as it should thorts pls

I received error code "P0170 Fuel Trim - Malfunction (Bank 1)" shortly after changing both downstream O2 sensors. Could it be a bad O2 Sensor on bank 1?

Graeme in OZ UPDATE !!!!!!!!! This weekend may have found my issue, while checking PCV valve I noticed there was a small rubber pipe connected to the valve cover then led to PCV valve the push on connection was very easy to pull apart so I installed small hose clamp to secure it. I then connected my newly acquired Foxwell NT301 OBD2 Engine Fault Code Reader Scanner and found the instrument gave me a green tick indication when connected where as before it was an orange triangle (Yellow indicates there's an issue that has been solved). The Short Term Fuel Trim readings now look better showing around -2 banks 1 & 2 LTFT is about -9 but from what I've read this takes time to adjust after problem is resolved. Will need to take her for a few drives to see if I have solved the 13 Year issue !!!!!!!!. Graeme in OZ

Amazing. Thanks so much!!!

Thank you.liked this class. Learning lots.

I have this problem since two months now, which literally drives me crazy! About the symptoms: Very noticeable jerking, stuttering and starting difficulties when the engine is warm/hot. Sometimes the engine won't even start at all. Engine has starting-problems. The problem only occurs after the engine has warmed up and ONLY when starting/idling. Error only occurs after about 10-15 minutes after the car has been shut down. ISTA says P0172: system too rich; which means that the DME either detects too little air or too much gasoline. So to reproduce the problem: 1. Car start perfectly when cold, not a single problem or vibration! 2. I drive half an hour normally (City/motorway) 3. Shutting the car off. 4. Waiting 15 - 20 minutes 5. Restarting the engine 6. Either the car starts with huge difficulties (sputters, shakes, jerks) OR it stalls and shuts off immediately. So far I've replaced: Intake manifold gaskets. (Even though this can't be the cause of the issue, since in the event of an air leak the DME would detect too much air and not too little (or am I wrong there?) 6x injectors 6x spark plugs 6x ignition coils MAF 2 O2-sensors 2 MAP sensors Air Filter Engine oil change (with filter) 2 Camshaft Positioning sensors Cleaned both Vanos-Selenoid Crankshaft Positioning sensor I'm struggling to understand why an intake leak would cause a rich mixture?!ISTA says I have to check the Intake system... (I already checked 3 times!, although without a smoke-tester) Wouldn't an air leak cause a lean condition since unmeasured air enters the engine? OR The ECU demands more fuel because there's an intake leak. In other words; since there is unexpectedly more air in the system, the DME has to spray more fuel to compensate the unmeasured air; hence "system too rich"? Therefore the ECU recognised that more fuel is sprayed into the engine as usual and because of this the DME says: system too rich? ----------------------------------- Intake leak = More (unmeasured) air requires more fuel to be sprayed into the engine to compensate the "unknown" air, therefore "system too rich" --> Because the DME has to put more fuel then normal? OR Intake leak = More (unmeasured) air requires more fuel to be sprayed into the engine to compensate the "unknown" air, therefore "system too lean"? --> Because there is more air then normal? Am I losing the plot here? Both make sense, but which one is scientifically and physically right? I'm desperate and on the verge of a mental breakdown

hi, i,m new subscriber, great video! my car is nissan skyline 300gt 2002 year. lambda is 1,017, but CO is very high 1,57-1,76. what is possible causes? thanks!

Hey, thanks for the video. Very helpful. My shogun sport v6 has codes P0170 and P0173, both fuel trims. It seems to be running fine except a slight change in idling speed every minute or so (from 1000 to 700ish RPM). The previous owner has changed an o2 sensor (top of cat) but the 'check engine light' is back on. I've unplugged the MAF sensor and the idling speed increases, then goes back to normal once plugged back in. That's all I've done so far. Any ideas appreciated! Bit of a head scratcher 🤔

I need y’all‘s help I have a 97 Toyota forerunner 3.4 L that I’m having a major Rich issue and every time I try something it does not work. I have replaced my O2 sensors, engine coolant temperature sensor, injectors, mass airflow sensor, fuel pressure sensor and about to change my fuel pressure regulator,fuel filter all vacuum lines as well as PCV valve, throttle position sensor. I got a new IAT but the vacuum tube on it fell off so I put the old one back on. I’ve done a compression test and all are within spec new spark plugs and coils and have swap the wires out with some wires that I had from before just to see if the ones that were on it are bad. The plugs are the dual tipped plugs that are recommended. There is a colder intake on the system which has been working fine on it for the past five years. All my fuses and relays check out yeT I am still getting a -20 something for my short term -17ish for my long-term. I get a misfire cylinder one error code but when I look at cylinder one misfire Count is less than 10 in the span of an hour. If I hit the throttle short term will zero out for a few seconds and then slowly Drop back to -20. When the engine is cool and I start it the mass air will be pulling close to 10 psi but after it Gets done with its warm-up cycle it drops down to around 3 sometimes even 2 1/2 psi. My injection it’s so low that the car almost stalls out somewhere in the upper 2gpm area. And when I first start the vehicle after it cooling down for a while it takes a long time to start. Somebody help me please

I have a P0171 lean code for a Corolla 2001 with 197k miles. My Long Fuel Trims are 40.1 %. Could that be that i have a clogged fuel injector? I cleanned my MAF sensor, change my O2 sensors, change All Gaskets. Still have the code ON. Any advise would be great...

I had a throttle body relearn done to fix the constant problem after cleaning it. I think I may have made a mistake I did a fuel composition reset using X431 on my 2011 5.3 flex fuel suburban , currently have e85 in my tank. Now I have 2 code errors p0171-00 and p0174-00 fuel trim lean system lean bank 1 and 2. Did I do something wrong? I would appreciate any help what to do and proceed. Thank you

Great explanation on volumemetric efficiency

Hi guys. I have watched a ton of your videos and loads others and still stuck. i am working on a 2001 corolla.1.8l..174,000 miles. code is a po171..lean bank 1. I have high fuel trims. shrt sometimes maxs out at 19.5% and lngt is always high 15 to 20 % +. Trims will drop at 2500rpm...maf reading about 1.8 at idle and rises up to 8 or 9 at 2500rpm. I have done a smoke test and could not detect a leak. o2s b1 wave looks ok, average about .485, and does switch wave from .1 to about.800...I dont think its fuel delivery because the trims drop at higher rpms. . vacuum tested about 21 with needle bounce, possible valve guides leak? Freeze frame showed Fsystem..cl...ECT 179Deg,,, shtft -2.2,,, lgtft 34.4%,,,rpm, 710,,,,,IAT 95Deg,,, I am about to start throwing money at it. not sure what else to do. i need the car fixed to pass emmisions for my daughter to start college. Live in tx and have to pass emmisions. Looking at new maf,o2sensors. Already changed pcv and air filter and cleaned the maf.Help !!

So if my 03 Toyota Corolla S is giving me P0420 and p0446 and p0440 and p0171. Should I replace both o2. The car has like 350000 miles lol so I’m guessing everything is old or shot and needs to be replace... thank u

I have an old school truck a 89 s10 pickup with a 2.5 liter engine and a manual transmission. I have replaced everything but the tps and map sensor. New exhaust including the cat. New gas tank and fuel pump. Odd thing with this truck is it hates fuel pumps. Person I purchased truck from also replaced the fuel pump and hanger assembly. I live near Harrisburg Pa and I can drive for about 15 minutes and the truck runs fine. It's like a switch was hit and the truck starts to run like crap. When I put my foot on clutch the engine almost stalls. Sometimes it does. The engine shakes violently and I almost didn't make it home today. How does the engine go from running great to running like crap? I wish you could give the ecm a error. I have a snap on red brick. I got codes 23 22 13 34 and 32. Before I left this morning I unplugged the o2 sensor hoping to keep it in open loop. The only thing I don't understand on the red brick is the spark count and block learn and interrogater. I know the o2 sensor works and it's not lazy. What is causing this? The way they engineer stuff is why I don't do this professionally any more. The stupid stuff engineers come up with. Don't know how you deal with their stupidity!

Well done good vid again on the page where you showed 450 MV rich and lean this is only on narrow band 02 sensor (nothing on the page about wide band ) I struggle with wide band still learning .great vid guys loving it 👍👍👍

Question I replaced the intake manifold gasket and Throttlebody gasket and replaced the egr valve and evap and still getting a lean code what can it be

Thank you. Really appreciate it

Hi I have a 2000 honda accord 2.3L and it over heated so bad I had the Spark plug wires melted on the Spark plug so I fixed all that and replace my spark plugs and wires and I replaced the thermostat and now the car won't turn on unless I step on the gas alil so I connect a OBD REAMER to it and the codes it brings up is cylinder 2 misfire and cylinder 4 misfire and car running rich what can be the problem? Thanks

Thank you for the knowledge

I have a 2002 Dodge Neon SE, P0172 code with 160,000 miles. No vacuum leaks, new plugs, new map sensor, new IAC, new O2, and new injectors. Still have the code after reset. Can you guys help? I`m ready to push my daily driver off a cliff. Fuel trims are pegged at -25. codes were reset after each part was replaced. Car will not pass Pa. inspection with MIL on.

Hi great lesson, my LTFT is at 0% since I checked a few months ago, does that mean my trims are really good? my STFT go's from -6% to +6 and bounces up and down between the two . is that a good thing ?

I have a 06 VW beetle 2.5l. I have a p0171 code. Did a smoke test and no vacuum leak was detected. Car is running rough on idle and reading short term 25% and long term 20%. I changed the maf back in July and it was running great until now. No maf code or anything when it started to have problems. I was told to replace the 02 sensors. Please help.

Hello I have this code PO134 on a 96 Buick Century 3.1 V6. I have a new O2 sensor in it can help me with this problem ?

Awesome video my 2005 mercedes e320 had a lean code what I fund is missed placed snorkle i clear the code I hope everything is fine now thank you

My 03 F150 Lariat 5.4 keeps giving me a P0401 code and negative trims around -12 to -16. I have replaced EGR valve, DPFE sensor (and hoses), EGR tube, EGR vaccum solenoid, IAC valve, plugs, coils, intake manifold, fuel injectors, and EVAP purge solenoid...

CEL TOO RICH TO LEAN AND MISFIRES. I changed maf sensor n air filters. Light still on Then Changed all Fuel injectors light still on. Changed all spark plugs and all oxygens sensors light still popped back on. What could this be please help! I’m tired of fixing stuff that probably wasn’t the answer. Last hope was the oxygen sensors once they were out they were fried bad . Once I clear the codes rev up and drive it handles fine. But once sitting idled it sputters

I've have learn a lot from watching this video thanks guys Just one question. My car was running lean on bank 2 due to a vacuum leak. 21 short term and 16 long term during idle Well I got that fixed and now its running rich each time I accelerate. Like sometimes 8- to 13- on the long trim and 5 to 8- on short trim. The fuel trims are okay during idle Is there anything I missed? or is it normal for the car to run very rich while accelerating it.

P304 code & P0174. Have 2009 Ford escape 3.0l. How does the miss fire possibly cause my lean code?

Hi again.. I have another question and is regarding my car.. I have bad CO, HC emmission problem. Mine has an ECU. And if my O2 and other sensors which cause rich conditions have problems it will show as an error code ryt?? in my car no such error code exists.. does it mean that i have problem other than sensors? or can sensor performance be poor which cannot be identified by ecu???

Hi I need some help I have p1137 add fuel bank 1 rich. I have put all new parts to do with that code still comes up.

What about a failed spark plug boot or leak in hose to evac.

hi.. again this is vidura.. in my car there is only an OBD 1 port which is under the bonnet of my car. This is because OBD11 port which enables the scanners to connect came after my car's generation. Is there any way of checking fuel trims from my car?

excellent class .

so I have a p0171, p0172 and a p0300 code on a 2002 gmc sierra denali 6.0 vortec. rich and lean in bank one with multiple random misfires, intake manifold gasket?

I am facing issues with p0172 ... I changed fuel injectors, air fuel ratio sensor, o2 sensor downstream, ignition coils and sparks

I got a 08 Hyundai Accent with. P2096 code. Changed both upstream and downstream oxygen sensors and I still get the code back......any suggestions?

Do you have a suggestion for this? My VW Golf TSI 2008 (BLG engine code) has a Fuel System Too Rich error. When the exhaust emissions are measured at idle all is fine, but at 2500 rpm the carbon monoxide is way too high. So the car failed inspection today. The error code is actually not present at the moment. I deleted it months ago and it didn't come back. So I forgot about it until I was reminded about it today due to the failed inspection. The error code was P0172

wow your the best man, good job

My 2010 Chevy malibu I have the po172 code engine will nit start the oil has gas in it the MAP sensor checked out ok any ideas that would make bank 1 rich

So nice to see other guys my kind of age who’s lost when it comes to carbs! I know the basics and I know it’s ‘simple fuel control’ buts for me efi is

great info!! hard to follow with the difference in time between video and audio!!

I have a 06 range rove hse 4.4 jag motor, I was getting p0171,p0174..it ended up being a bad fuel pump so I replaced and those codes went away now I'm getting p0172,p0175 I have no idea any suggestion?

Please I need your help with some advice. I recently changed my fuel pump and filter on my mercedes w164 and I noticed the car usually jerks a little when I come to a complete stop at streetlights or traffic. So I scanned and discovered I had a "large leak in EVAP system" and "system too rich" error. I have checked my fuel cap but the error has persisted. I kind of feel the covers for the fuel pump and filter may not have been properly fit. So I plan to re-tight it again. My question here is this: Can a "large leak in EVAP system" error cause the "system too rich" error???

It's a great tutorial!!

I have a 1998 mercedes c230 with a P0170(running rich). I swapped out the maf sensor bc it looked like the easy answer. I have a cracked air filter housing will this be the source of my issue? Next would be to clean inspect my throttle body. After that im going to change my fuel filter and injectors. Car was in and out of storage so it may be rusted injectors? P0170 is the only code present.

Ayy I got a GTO to lol but I’m watching this vid for my Ctsv

I have a question i have a 2013 kia rio. LTFT is at +30 and STFT is 0. I checked for vacuum leaks and have none. Oxygen sensor reacts to fuel being sprayed into intake. I bypassed fuel system with terra clean machine and no change in trims. my question is could carbon on the intake valves cause this much fuel trim correction? this is a GDI engine.

I've got a Ford Focus 1.6 petrol 2006 mark 2. It's long term fuel is minus 10. It's long term negative numbers have been slowly been going more and more negative. There is no engine light on and seems to run OK. Where do you suggest looking? Thanks :)