Short Term and Long Term Fuel Trims Explained 

this is one of the best explanation of short and long fuel trim I've seen on RU-vid. great job explaining it.

I've been a dealership technician for over 20 years and have been working for myself for the past 10 years now and I have heard many people who thought they understood long and short fuel trim but after listening to their explanation I realized they didn't have a clue. Without a doubt you have simplified fuel trim and explained it in a way any decent tech would understand. Most technicians are lost if they don't have a DTC to point them in the right direction when diagnosing drivability concerns. If you have a good understanding of how fuel trim works why and what will make the values change you can diagnose most drivability problems quickly. Thanks for a very informative video worth watching and please keep more like this coming.

I rarely leave comments. But the quality of this video blew me away. So informative and well put together. I'm walking away feeling confident I can repair my misfire.

The best explanation on the internet of long and short term fuel trim. Other channels give more detail and end up confusing me. You cut to the basics and explain them in a way that is easily understood. Thanks a lot!

I'm a life long D.I.Y. guy and I watch a ton of these videos for different views and perspectives and you my friend are worth the subscribe.. keep up the good work and keep the videos coming..

EXCELLENT video! I've been DIY repairing my car for about three years now, but fuel trims have been a mystery to me. Even after reading several articles, I just didn't get it. A few years back I dyno'd my car to check performance, and a lack of performance led me to monitor my AFR and found a lean condition. I should have looked at fuel trims, but didn't understand them. I changed the fuel filter on my 100K mile car as a stab in the dark. My AFR returned to expected and re-dyno'd the car with expected whp results. After watching your video, I'm going to check my fuel trims soon and I'll have confidence next time I run into a lean/rich problem. Thanks!

I spent a whole week moving between clips and books in order to understand the fuel trim and instead of understanding the complexity of the matter more, I thank you very much for the clear and simple explanation

I’m NOT a technician and after watching this video I can teach others what is the Air fuel ratio and Short/ Long term fuel trim 👍👍. Thank you man!!

AWESOME VIDEO, you taught me exactly what I've been needing to know and understood it the first time around. Glad you added the example numbers

Well executed video with a good mix of practical and theoretical.Looking forward to the super duper smoke machine !!

Thank you! I work at a parts store, and one of our regulars called last night and was asking about fuel trims, and I told him I would look into it for him. Your vid was the first to pop up and WOW! A GREAT help to me. I think I should be able to help him now. :)

I have been working on my own vehicles now for almost 40 years now. in that time I have come to know how several of the emission control components work, at least enough to make necessary repairs. I am one of those people that needs to know the "big picture", once I understand where I am trying to go, it becomes much easier to find the path to get there. in the first few minutes of your video as soon as you explained the mandated air/fuel mixture ratio , for the first time in my life I began to see the big picture of emission control. And by the time you finished explaining the role of the O2 sensor , I felt like I finally knew what was going on, and how all these components play together to acheave this one simple little goal, 14.7/1 (the golden ratio of combustion) feel free to use that term if you like, LOL. this was the best video I have ever seen on RU-vid. Your explanation was simple but thorough, your graphics illustrated what you were explaining very well. And the progression, and order of your thoughts were logical and methodical . thank you very much for expanding my knowledge, Leonard Oh, by the way do you think the cigar idea has any merritt?

Dude - FANTASTIC INFORMATION! Concise and really shows the why behind the numbers. Great explanation! And yes the Essex engine in general had issues with internal intake seals and the isolator bolts. I think you nailed the diagnosis

JUST BRILLIANT! I have watched a lot of auto channels and this one just beats them all by light years!

the way you explain things makes it so easy to learn, thank you for this.

Thank you for the excellent explanation. Fuel trims are much clearer now

Your explanations are some of the best out there. Thorough, simple to understand, humorous and no BS. Thank you! I learned a lot from this video.

Best video I've seen on the subject. Simply explained without getting off-track. Very helpful.

Great video! Learning alot. I believe your right. the exhaust restriction will cause more of a rich condition. Since unburn gases are restricted by a clog cat unburn fuel will re enter the cylinder unbalancing the mixture. An open egr or clogged egr can cause a lean condition.

I love the white board of learning. Your good at explaining the systems and process of troubleshooting.

this is the best explanation of short & long term fuel trim thank you, sir! keep up great work! keep it simple!

By far the best fuel trim/,O2 sensor video on RU-vid. Some people are just really good at teaching and explaining

Finally you got me on this one. I got a new scanner that I can get me a bunch of info and watching over 10 video explaining about fuel trim. You should be teaching at school sir. Thank you a lotttt

It was super valuable to see what the O2 sensor readings should be doing! Helped me determine my O2 sensor is bad.

This is the exact information that I needed and have been looking for. Thank you so much for sharing.

this is by far one of the best st/lt fuel trim videos on yt. thought i had a grouding issue, but after scratching my head for a minute i looked at the data. damn, saved me $$$!

getting back on the tools after a 4 year hiatus these videos are awesome for rebooting the brain!!

concise and helpful; able to convey the concepts of fuel/air management and trouble shooting efficiently

I wish I had college professors that could explain things as well as this guy. Great video.

Thanks so very much for explaining the short and long fuel trims! I have never been able to understand this until now! You are an excellent teacher!😀

Best description for fuel trim, watching the video a few times helped me pass engine performance ASE 👌

So glad I found this. This will help me to understand the data logging that I have started to do on my truck. Thank you for taking the time to explain how Long and Short term fuel trims work and how they are supposed to look.

Thanks a lot. i will check my car's fuel trims tomorrow.

Great explanation of how it works! This helped me determine a vacuum leak on my car with a persistent misfire.. Now i just need to find it!

Out of all the videos I clicked on for a simple and straight forward explanation this was the one 👌🏼 ty sir!

Aloha R&W, I recently found your videos and find them very easy and comprehensive to follow. You have good explanations with "why" which all comes together to make sense. Good video shooting too. Keep up the excellent work! Looking forward to seeing more from you. Thanks for teaching us.

Dude you're amazing, all your videos are helpful, thanks for the infos.

Great explanation and very well explained. I thought I understood the fuel trims, but your explanation.gave me some new ideas.

Great video and superb explanation. I have the exact same readings on my LS3 engine. Suspecting an inside vacuum leak within or around the plastic intake. Again, thanks a lot.

Wonderfully informative video. Clear. To the point with no bullshit and honest disclosure. We need more like this.

Actually on higher rpm the ratio gets a lot richer than 14.7:1 , especially with turbo engines . This is needed due to the higher heat in the combustion chamber . Fuel is like intercooler , aqueous intercooler . This helps to prevent melting the pistons . For a example , Mitsubishi Evolution have around ~11.5:1 ratio , and the tuned ones can have even 9 or 10:1 , so the mapper can be on the safety side :D P.S I'm about to make my own smoke machine , but going to wait you to steal some ideas from you . Great video as usual !

Great video , well explained for a old school mechanic , helps me understand what's going on . My S10 2.2l is around 14.6+ lean ,I just put an alternator on maybe I knocked a vacuum line loose , thanks great job👍👍🏁

I know this is an old one, but it is still a great one. I am a long time subscriber, life long auto mechanic (retired), and 63 years old. I learn something from you almost every one of your YT I watch. You are a very smart and intelligent young man. Thanks for all you do here on YT. And yes, how do we make our own Smoke Machine. "South Main Garage (auto repair)" is a lot like you, but for us regular folks working at home, we don't have all the cool tools he has. He has a smoke machine (first I learned of one). As a guy working in his garage and driveway, how do I make my own smoke machine??

One thing to note when it comes to fuel trims and vacuum leaks that *very rarely* gets noted - you'll most likely only see high positive low rpm fuel trims (lean condition) with a vacuum leak if your engine is MAF based. If you happen to have a MAP based engine (as in engine that doesn't have MAF sensor, also known as speed density engine), your fuel trims will likely be perfectly normal. That's because with a MAP sensor, you more or less can't have unmetered air in the engine - so even if the air is coming into the engine via a vacuum leak, it will be metered by the MAP sensor just fine and therefore will *not* cause a lean conditon (well, it still might under certain conditions, but it's much less likely). But your engine performance will still suffer due to the vacuum leak. Knowing this might save you a lot of headache trying to find a vacuum leak that actually isn't there if your car has a MAP based engine. Trust me, I know ;)

This is great info. Thanks

One of the best videos I have seen about fuel trims, awesome video man. Looking forward to future ones 👍🏼

This is spot on exactly what is going on with my Chevy S10. Thank you so much for making this easy to understand. 🤙

I have question, could placement of an o2 sensor effect fuel trim readings of the engine, for example of the factory manifold had the o2 sensor at the top, and the aftermarket moves the o2 sensor down underneath the vehicle

Yeah, please make a video showing how to make a DIY smoke machine!

Thank you for this video, it was really helpful and self explanatory, I was able to determine what was going with my old civic, turned out my map sensor wasn't working right and Idle control valve kept open for longer and my idle was dropping below normal. My long term fuel was at 12.5 % while myshort term was at 28.5% i replaced injectors just to be on the safe side and everything its normal now , long term is at 2.3% short term is at -3.2+% thanks for your videos keep up the good work 😀

Very good, you explained this situation like a pro! Thanks for the insight, it really is VERY useful information for the DIY's in the world.

get a cheap cigar disconnect the vacuum hose on the brake booster blow smoke in and see where the leak is and that's how you make a smoke much

That was a great video. Thanks. I learned a lot. The problem is most likely the isolator bolts that hold the intake plenum to the intake manifold. The rubber gets old and they leak. Ford has a TSB out about it.

You are DA Man ! All the issues i have searched and You diagnose was the only one that clearly made sense ! I have the same condition on my 2011 335i and LTF is running at 15% while my O2 's are doing good so Intake leak for sure ,I suspect Inlet at the inter cooler ,the cheep plastic and one bolt that holds in the rubber ring ,Needs upgrading ! Thank YOU okay update apparently my long fuel trim spikes when I apply my brakes and come to a stop otherwise it runs around 3% and everything else looks great so I had a M3 with a grommet brake booster issue looks like this is notorious happy hunting all this is a place where most people don't realize

Great video! Very well explained. I now have increased knowledge to diagnose my car issues.

This is amazing, perfectly and clearly explained. I’m hoping this can help me narrow down my rough start condition on my 07 cobalt!

YOUR VIDEOS ARE THE BEST!! My problem isn’t exactly like this but I watched anyways because you’re seriously great at explaining stuff. I’m getting codes 0171, 0174 & 0301 and this mechanic says I need to change spark plugs and replace all these pcv and breather hoses and wants to charge me $900.. but I feel like he’s being shady because I oiterallly changed EVERY spark plug the day I took it to him LOL. I’ll keep watching your videos hoping to find the right one for me :) thanks man

I knew how it works from school, but could not remember (and could not easily find online) the 'normal' values...:) I'm glad you gave us the values. Thanks I just subscribed

Great vid. It is not how much you know, but how good you can explain what you know. You Sir.. Are great at both!

Wow great explanation, UTI’s instructors couldn’t explain such an important issue! Instant subscriber

So professional and to the point, with proof more professional. Thanks. I got a Mazda CX7 with the same mystery, but causing a misfire on cylinder 2 and 3. I think with exhaust leak it's gonna be solved

Older video but subscribed because of it. Really easy to follow and straight to the point. Great video

Fantastic video. Clear and concise. Gonna send customers to it so they better understand what I’m doing and why.👍

Awesome explanation of ST and LT fuel trims. Thanks very much!

hi, this is the best tutorial on fuel trims and O2 sensor. Thank you

lots of good information. i will try this on my car. i am facing issue like rpm drop and vibration at idle, struggles when clutch is engaged in first gear. cleaned maf and egr but no success. I think LTFT will help me to get to problem. once again great video.

Thank you, Sir. Very helpful. My old Honda with over 200K miles is showing 4.7% long term trim and I didn't have an idea what the heck it is.

thank you for explaining how the short and long fuel trims work.

Thank you very much for sharing your knowledge. Great explanation with numbers and everything.

Thank you for your awesome explanation. Im diagnosing a p0174 on my 04 F150 4.6 that has 5k miles on the new engine. I bought a scan tool with live data to see if I could find the issue. I've checked for vacuum leaks with spray and used a vac guage and am getting steady 20 psi vaccum off the manifold and the brake booster. I've cleaned the maf and CEL keeps coming back. Will be reading live data once it comes on again.

Can't wait for the smoke test video. I tried making one with a fog machine but couldn't get enough pressure from it to push the fog out of an open vacuum port on the manifold.

Great explanation, helped me diagnose a few tickly problem child cars! Thanks cheers from Australia

Just by color coding the air/fuel and rich/lean conditions, you have cleared up so much confusion.

Great explanation on short/long term fuel trim via example!

Thanks for the information. You explained it very clearly and will help me diagnose a lean condition.

Excellent Video. Should be able to pinpoint the problem on my '03 Six-banger Mustang now. Didn't have the relationship between the readings and the Injectors until now. Thanks !

man you explained this way better than my instructors. thank you

This help me out a lot from learning how to read thru obd thank you!!! Btw i saw your video on smoke machine. Instead of using rag and oil. I used smoke bombs (fireworks) produced more smoke without using compressor due to high amount of smoke pressure from smoke bombs

very well explained, this is the info I have been trying to find and learn. thank you

Your just so good at teaching! Thanks for all your hard work!

I just got 2 years of automotive tech school in a few minutes! Thank you! You think you know about cars then you watch something like this!

Very well explained, understood every bit, will test my Toyota 1MZ-FE engine out. Thank You

Great video, find myself coming back whenever i have an issue.

Great video bro, very knowledgeable and you explain very thoroughly. Much appreciated brother

You are very clear with your automotive suggestions, I appreciate this a lot! I have a 2004 GMC Canyon, automatic transmission, 3.5 litre 5 cyl. The LTFT is +12 to 13 %. I decided to use my new scanner tool to show me live data while I was driving my truck, to see if I could spot any strange or concerning anomalies. Nothing showed up if I was just idling in the truck. I did find something strange while I was out driving on the road test. I set the scan tool to show me only 4 statistics: STFT, LTFT, O2 sensor #1, and O2 sensor #2. So on the drive, the readings of the 4 statistics would fluctuate up and down as a person would expect. Then suddenly while driving at about 90 km/hr, and 1750 RPM according to my dashboard readings, the scan tool all of a sudden showed that the engine was locked at readings of STFT 0.0%, LTFT 1.6%, Sensor #1 at 0.005 Volts, and Sensor #2 at 0.40 Volts. It was my understanding that the O2 sensor # 1 would always fluctuate up and down from zero to 1 volt while driving, if working properly? The scan tool showed these frozen/stuck engine readings while driving at 90 km/hr, and did the same thing at 110km/hr. The fuel trims and the O2 sensors were frozen in the readings, for about 5 seconds to 10 seconds, without changing. Sometimes the voltage of the O2 sensor #1 would very minimally change to 0.015 v, and O2 sensor #2 to 0.050 Volt. I have been experiencing a lack of power and hesitation, at these exact speeds, in the weeks prior, so the scanner findings make sense, I guess. What do you think about this? Any of your experience and knowledge would be appreciated for sure! I’m guessing if it is possible that the lock up mode of the automatic transmission has something to do with it? To me it does not seem logical that while driving and the transmission enters into lock up, that it would ever cause an O2 sensor reading to freeze and the Short Term Fuel Trim to freeze accordingly? When I am at idle, the O2 sensor #1 seems to work properly…it fluctuates from say .850 and all the way down to .015 as a rough explanation. Is there anything that would cause both O2 sensors to die at same time? Oh, I forgot to mention the odometer is at 418, 500 Kms. Well, thank you for all of your informative videos!!!

always love the informative videos! learn so much. Thanks for everything! Keep it up!

This is a flippin brilliant video! All that info in a short video. Thanks RnW !! Bad Fuel Mileage but no CEL. I'll do the check on the car tomorrow. Brilliant!

Bro just wanted you to know your videos are always on point. You da man

Fantastic Video made it easy for me to understand possibly the best explained video I've seen in a long time. Thank You very much

Wow, absolutely great video. Clear, concise, and interesting. Thank you so much.

Great video and explanation. After watching a few videos I've subscribed to your channel (I subscribe to very little). Keep up the great work, good info and to the point without overly complicating things.

Thanks for better understanding of O2 sensors 👍🏼🙏

Mannn....you explained in detail and i like that. Makes my job easier. Awesome vid looking forward for more.

R&W, thanks for taking the time to make this video. It would be a little clearer to understand, if you would identified the O2 sensor sending the signal to go rich/lean i.e., O2 sensor 1/2.

you really make some excellent videos. He would be a great auto instructor in a trade school...thanks for these videos.

You obviously know your stuff. I need to know your stuff. Thank you for sharing what you know so that can happen. Already subscribed.

Great tutorial well explained. I suspect I have a dirty mass air flow sensor.

This is among best I've seen on stft and ltft.

Best explanation of fuel trims I've seen. Thanks man!

Man, Your videos are AMAZING! So easy to understand, very good instructions, nice demonstrations. Wondering if maybe you could do a video on transmissions and shifting? (Hard shifting, laggy shifts, shudder, etc)