Using Fuel Trim to identify type of misfire (can it be done?) 

Scanner danner is a blessing to all mechanic's I'm a ASE master tech but I get rusty and his video's help refresh my memory as well as learn new techniques. Thank you MR Danner. ✌️

There are a number of excellent automotive teachers on youtube, and I have learned from and have been helped by many. But you, Scanner Danner, stand alone as far and away THE BEST. I look forward to buying and reading your book, and to subscribing to your premium channel. I have no doubt both will be worth every penny and far more.

Thanks for your input. If I could get past all the bashing I would. I agree with you about some misunderstandings I may have with the O2 sensor and how it actually works. When I can put all of this together I will do a follow-up video to this. The main point of this video was to show that an ignition misfire does not cause a rich condition and negative fuel trim numbers. I would like to hear about your bad cylinder negative fuel trim scenario. Thanks again for your time and comment

After 10 years I'm getting back into auto mechanics and Diagnostics... Thank you for the teachings💯

Thank you very much for the compliment. I am glad you like them. Keep watching, I am working on more everyday. Paul (ScannerDanner)

I just stumbled across your videos for the first time and it is very refreshing to find someone posting videos on here that actually knows what they are talking about! And it still kind of amazes me that people forget that o2 senors measure the oxygen and they forget, or fail to realize, that the oxygen is no longer being burnt so even though there is excessive HC in the exhaust there is also a raised O2 level now. But anyways great videos and keep up the great posts.

Thank you for the comment. The Picoscope I am using is the 4000 series automotive 4 channel. It is awesome isn't it? Nothing wrong with the MT2500, you won't need a Verus if you got a Pico. Look into the Solus Ultra when it comes time to upgrade the scanner.

You had it right in your comments on the ETCG video. I just felt the need to do this to clear the air for those that think an O2 will go rich with an ignition/compression misfire, from unburned, raw fuel in the exhaust. As you know, as misfire will drive the O2 lean regardless of the type of misfire. Thanks for the comment and great advice to the other poster on ETCG.

Paul, I had a citeron c3 1.6 8v in yesterday with inj no 4 open circuit DTC stored. Checked the STFT and that was 23%.checked it on the gas analyser and the HC were very low which confirmed that the injector wasn't working. Removed the plug and found corrosion on the terminals. Checked resistance 2.2 against the other 3 of 13ohms. Replaced the injector which brought the fuel trims back in line. Your videos help me so much! Keep up the excellent work!

This is not a difficult concept to understand. Especially if you know that an oxygen sensor is exactly that, a sensor that reacts to oxygen. The term "rich" when discussing fuel trims, and fuel mixtures in general, is probably where most of the misconceptions come from. People I've worked with don't realize that rich isn't rich. Rich is oxygen deficient. Oxygen deficiency can occur for many reasons but at no time can an oxygen sensor determine if a system has more fuel than it's supposed to have. Great video.

This is (another) illuminating seminar, Paul. It also gets to the semantic issue of what we define as rich and lean -- a little confusing when you think about it....

Great vids man!!! you're the best!!! when the coil is unplugged, the mixture doesn't burn, hence the Oxygen stays raw, and the O2 sensor picks it up and tries to add more fuel to compensate. When fuel is cut, you have raw more Oxygen (taking the space of fuel as stated by yourself) so higher trims... Cheers!!!!

Logical and intelligent analysis.. About note the height of fuel tirm in the case of no fuel misfire compared to the case of Ignition misfire.. Thanks Scanner Danner

Yesterday, one of the techs in my shop was diagnosing a surge at idle and an intermittent stalling issue. He had codes for a small leak and oxygen sensors. He was trying to command the vent valve close to see if it could see an increase in fuel tank pressure, but at some point saw negative pressure. He was trying to associate the vent valve with the surge, but I could not see the surge as a symptom of a restricted vent valve. Today, another tech looked at it and checked one spark plug and the electrode was eaten away. Guess what, the LTFT and STFT were single digit negative. Logically, they should not be negative, but they were. Love your videos as always.

I work in developing software for ECUs and I like how ScannerDanner presents such topics in simple and interesting way.. Even after 11 years this channel is my favorite. Great job

thanks for this information..I've always been led to believe the theory about lean and rich misfires, thanks for the clarification

Excellent excellent subject matter to cover.. Now I see the error in my ways of using fuel trim to diagnose misfire.. Thanks Dan u are the greatest!!

Thanks! Some GDI systems are certainly on my radar.

you didn't need help, you got it :-) One thing I was trying to cover in the video was that ignition misfires DO NOT make the O2 go rich and cause negative fuel trim numbers. There is another guy here on youtube that "trains" people that was claiming that to be the case.

I'm going to jump in and answer this since no one else has yet. I don't know what you mean by Internal v. External problem is. As his name implies, his video is about not using the scanner to diagnose a misfire. Quick checks would be examining the spark plug, wires and the boots, checking fire and voltage to and from the coil(s), checking for voltage signals to the injectors and feeling each injector to see if it is firing. Do a balance-of-power to find the faulty cylinder. (continued)

Thanks for clearing that up, Paul. Very good video. Even though the system went open loop on the narrow band, you could probably see the O2 sensor voltage PID and see that it was indicating lean. In fact, I wonder what the O2 voltage would look like on forced open loop system with an engine misfire? Can't wait to see your video on wide band and how the ECU knows the cat is bad on that system since it really can't compare front and rear o2 voltages.

Very well explained! Still interesting why the fuel trim goes up. It is much easier to remember things like this if you understand every detail

This is definitely Vital Information, that I was not aware of with the Fuel Trims. Keep the info Coming ! ! !

Thank you my friend, I will. Spent all day reading about the tomato :) Chemistry is difficult for me to understand, especially the stuff that is being talked about in a lot of the tomato threads.

I just used this to find a stuck injector on a 3.4 gm. Buddy had a miss. He replaced plugs/wire. Still missing. Used fuel trims to see it was running lean, then scoped the injectors and looked for a missing pintle hump. He had been driving like that for 6 months cause he couldn't figure it out. I fixed it in 45 minutes thanks to Danners channel.

Mate your channel is awesome, you explained what the data was all about on the display on my Bosch 2220 found my no2 bank in my ford xr6 BA STFT was 30 it's idle has been really rough, also in my Ford AU I found inlet manifold Inlet gasket leak so I'm guessing it's the same with my BA as well, ruddy cars!

Thanks Danner great work, ive been trying to research this for years, God bless

Thanks ! This puts us on the best diagnostic path . Very helpful ! Now we know !! 👊🏼

Great video, perfect timing. Thank you again!

I am man enough to admit that i was wrong and you have shown me the light brother!i guess the reason i thought this was valid was because most of the cars i have checked have had fuel injector problems so when i saw the positive fuel trim i thought it was right.thanks again bro,GOD bless!

I couldn't have said it better myself! Nice job.

Hey, no problem bro. I didn't do this video to prove anyone wrong, I actually did it for me too as I had some doubts as to the result. When you get a chance, check out sections 4 and 5 in my book (now that you have it). These two chapters cover O2 sensors and Fuel trim in great detail. I think it will help back up this video.

I like this guy. I'm an ME (but not in the automotive industry), and I do ALL of the work on my '08 Acura TL Type-S. A few weeks ago, a spark plug decided to take a shit, which triggered the MIL, followed by a surprising loss of power and the embarassement of limping my "garage queen" back home with her sounding like a fucking golf-cart. And a top speed to match. During my _LONG_ drive back to the house I was imagining every scenario possible as to what the issue might be, how much it would cost and how much back pain I would suffer while digging down to the troublesome part. Bad injector? Bad coil pack? Some pissy connector somewhere? I kept thinking about how a misfiring cylinder would affect the AFR and how I could use that info to help me pinpoint and solve the problem quicker. I was wrong. Way wrong. I wish I had seen this video before I fucked around and wasted time trying to diagnose the problem based upon incorrect assumptions about how different types of misfires would affect STFT and LTFT! Some simple, old-fashioned troubleshooting led me to the correct diagnosis. Nice job on the video dude. I have been re-edu-muckated...

great video yet again thank you would love to see some wideband stuff if you had time.

That is correct. Although there is some debate in this field as to the O2 sensor signal being affected by hydrocarbons. I am not debating that at this time as I am no chemist. But what we can definitely determine from this video is BOTH types of misfires drive the O2 signal lean due to excess oxygen in the exhaust. With the injector (no fuel) misfire showing even more lean. One other thing, unburned fuel IS NOT rich. This is a very common misconception in this field.

I will check those sensors' connectors, thank you. Right now I am waiting for parts, we did a fuel pressure test and it does not holds pressure at all. Well, thank you, nice talking to you.

My car is running rich on one side of the engine bank 2, yet the O2 sensor on the same side bank 2 is reading lean. My right side catalytic converter is shooting out a lot of black soot. It definitely has a misfire because the MIL is flashing. I have changed out the O2 sensors, spark plugs, cables, cap/rotor, fuel pump, fuel filter, EGR, gas regulator, MAP, & finally the MAF sensors. Watching this video made me realize it might be clogged fuel injector making the O2 sensor read a false lean.

This is just the scanner portion of the Verus I am using. You can do the same thing with ANY cheap scanner you just may not be able to graph the data.

I think ScannerDanner is a great guy as a tech and as a teacher keep it up with this great work !!

You switched the position of the LT and ST fuel trim graphs for the second car. Made my brain go into open loop there for a second or two.

You said it man,depends on many variables like COP , wast spark,distributor,misfire code or not,how accessible is the cylinder in question,how easy it is to current ramp etc.But if a gas analyzer is available ,I think wold be my first choice like you did with Eric.I had a Cadillac with North star engine in my shop today.No crank.To check for voltage drop at starter I had to remove the intake manifold,The cable was fine so I replace the starter.So it depends on the task at hand.LOL Thanks again

Thank you very much

great advice thanks!

You are absolutely correct that neither ign. nor lack of fuel misfires cause a rich condition. The reason being that people forget that there is a full cyl. worth of O2 going in there also, and even with the ign misfire and unburned fuel, the O2 content far out weighs the amount of fuel. Both misfires cause a lean condition and will drive trims rich. You come back on IATN and you wont be bashed, I'll see to it. Remember, one can't feel, see, or hear emotion online, so don't take things personal.

Nice Russ! Thank you

Extremely informative!

Going way back in the library and still learning to think outside the box today

Genius and a teacher. Thanks

I like all of your upload man you rock.

great job man, had a similar disagreement before couldn't win lol... glad you covered this . had a couple question, your using a picoscope right? what model? been looking at some PC based scopes and read some good reviews on the picoscope... any thoughts? and no i cant afford a verus lol as of right now im using a old bench top scope works well and for scanning a snap-on mt2500 and AutoEnginuity proline + Mitchell 5.8 its not the best setup but gets the job done. thanks, will

P1132 is B1S1 indicates rich (this would cause negative fuel trim numbers. And P1151 is B2S1 indicates lean (this would cause positive fuel trim numbers). So one bank is running rich the other is running lean, Is it possible the connectors for the upstream O2s can be switched? In other words they are connected to the wrong bank. This can cause this condition. Also you may want to check that thing for a bad headgasket.

A intake leak will have high short term numbers at idle but normal at 2500 rpm. A gas analyzer will distinguish between the misfires. Both have high O2 levels but no HCs on the fuel injector.

working on it brother

thank you for the information ,,great book I love it

(continued from previous post) More quick tests are to use a vacuum gauge and checking the fuel pressure. Finally do a compression check and also a Leak Down Test. Too many mechanics out there will only look at the scanner trouble code and just throw parts at it until they finally replace the right one while making the customer pay for all of those mistakes mostly because they don't know what they are doing. Learn how to use all of these tools. (checking spark voltage should be in above comment)

I copied this from my book. Higher than normal fuel psi, Ruptured fuel psi reg. diaphragm, Leaking fuel injector, EVAP purge problems, ECT or IAT sensor problems, MAP/BARO or MAF sensor problems, EGR valve stuck open at idle, Fuel contaminated oil, Head gasket leaking causing coolant contamination of the O2, Jumped timing chain/belt

Good job, ScannerDanner!!!

Excellent information.

Fantastic !! Would be nice to see a gas direct injection engine in the shop. Seems a lot of engines are using direct injection and your students will see lots of these out in the field.

Hello your video is very important in the diagnosis of faults. My language is not English. I am using a translator to communicate with you, and have tried to understand what you say , but I do not understand everything. You can tell me the times of the video where behaviors are no fuel misfire and an ignition misfire on both systems please? Thanks.

Even that isn't 100%. I don't think there is one test that I use more than the other. Cylinder drop test, secondary ignition waveform, primary ignition current ramp, injector voltage and current, noid light, relative compression test with a scope, gas analyzer....heck I'll even use a spray bottle with water in it to find a misfire:) The one I chose to use will change for what type of system it is and how easy it is do get to the components.

I’m new here. I’ve been experiencing rough idle with my 14 jeep Cherokee 4 cylinder. Using the bluedriver scanner. I found the LTFT at 19% and STFT at -4%. Replaced spark plugs and injectors with mopar. Rough idle is still there. Cleaned throttle body. Still there. Now i gotta check fuel injectors. This video really clarified alot

First of all ,an excellent presentation.My car is running rich with O2 sensor 1 reads from 0.45 and shoots up to 1.2 v as engine starts.Could it be lack of compression on one of cylinder which cause rich on O2 sensor 1 .

How would the post cat O2 sensor look in these cases?

I have the same exact car. Love my RX!

Dan, I have a 2001 5.3 tahoe with a code p0300 code. Borrowed my buddies scan tool. Misfire starts only when engine reaches operating temp. I noticed my o2 sensor B1 STFT trim was -31.2. MAP reading is 12.6. Fuel pressure is normal. I just started diagnosing my problem. All emmision sensors are new. I can run live data with this tool. What are normal readings for this engine at normal operating temperature? Any advice would be greatly appreciated. thanks, Tony

Thank you for this video. I have something to ask you, what about a 98 Ford explorer with codes p1132 and p1151 that runs really bad, had the MAF and Ox sensors replaced, how would you go about using the Ft to troubleshoot it ? The STFT and LTFT are at -100%. Thank you. MAc

Its Paul btw and thank you.

Does anybody have the link to the video paul has where he talks about the newer theory about how a o2 sensor operates?

Dear Paull, As I was "smoking out" for any leak from the top of the engine, I saw smokes from the EGR valve (inside the EGR body, not the contact part to the intake). This is for my 2004 Honda accord V6 3.0L engine. What is the relation of such leak to the fuel trim for this MAP based Honda engine? Regular MAF based engine, we should see positive STFT, right? But for MAP based engine, the STFT will be negative, right? Should I buy a new EGR from Honda or the aftermarket? Many thanks & God bless, Hai

Hi Paul, At AVI on demand, instructors Ron Bilyeau and Bill Fulton are both sure that the type of misfire can be determined by the fuel trims: saying that a rich density misfire will cause negative double digit fuel trim numbers and a lean misfire will cause positive double digit numbers and also that the rear o2 sensor can can indicate types of misfire: high mv is an ignition misfire while low mv is no fuel misfire.

I have found a recent fault in my car involving the Chevy LS1 where I went to fill up with fuel 2 weeks ago,well it involves the check engine symbol appearing on the cluster,the fuel trim being Maxed out to 25% on both banks,the fuel volume tested at about 3 litres per minute,the fuel filter isn't restricted & the fuel pressure tested at 47 psi at the fuel rail. The conclusion I came to is that the fuel mixture is lean & the fuel pump is noisy due to being worn out,the fuel pressure regulator holds pressure but the pump fails to deliver the 55-60 psi required for the LS1 engine so I ordered a pump to replace that one which I'll do this week.

scannerdanner hyundai santafe 2.7 trouble code p1167 , b2s1 stuck rich even in closed loop, the same time stft is 24 . o2 sensor replaced also maf but no change. i m astonishing why stft is lean not rich. any comment?

Buena presentación.

@ScannerDanner Thank for the video. Could you help me make sense of these STFT and LTFT values. I have 07 ford taurus, for bank 1 STFT stays between +/-3%, but LTFT stays constant at about -12% at Idle. Bank 2 is lean at idle with +12% total fuel trim. When I increase RPM to 2500, STFT jumps between +/-6% and LTFT stays constant at -19%. Bank 2 improves at 2500 RPM with about +6% total fuel trim. OBD says cylinder 1,2, and 6 misfire. I have tested injectors, replaced the leaking injector already. What do these trim value mean? How come bank 1 is rich and bank 2 is lean? Thank you.

For sure and I have to say that most people think unburned fuel (HC emissions) is rich. It's not.

Another brilliant vide.. By the way how can I buy you book and ow much dose it cost, Currently I am studen

Could you diagnose if the misfire is caused by ignition problems or fuel problems by looking at lambda ?

not sure if i understand this.. both trims read lean because o2 sensor job is not to read the extra fuel, they read oxygen, and there is extra oxygen both conditions. no fuel or no spark o2 sensor will detect xtra oxygen? help here!!

I thank you for all your wonderful inputs and suggestion. Let me show you the tables of the key parameters and their conditions and they also answer your questions: After about 8 minutes at IDLE, no speed, RPM = 785, pending codes for misfires all cylinders (P0301-P0306) and P0300 (Random misfire) show up: 1. ECT = 176 2. MAP = 10 inHg 3. Spark advance = 9 4. IAT = 95 5. TPS = 13.3 6. STFT bank 1 = -27.3, bank2 = -2.3 7. LTFT bank 1 = 0, bank 2 = 0 8. A/F ratio sensor 1 bank1 = -0.246 to -0.269mA (before CAT bank 1) 9. Eqv ratio 11 = 0.964 to 0.972 10. A/F ratio sensor 1 bank2 = -0.019 to -.074mA (before CAT bank 2) 11. Eqv ratio 12 = 0.991 to 0.999 12. O2S2B1 = 0.825V (after CAT bank1) 13. O2S2B2 = 0.195 to 0.715V (after CAT bank2) 14. Load = 34.1 Then I drive the car, during ACCELERATION I see O2S2B1 = 0.3 to 0.6V (after CAT bank1) Per your instruction, I take notes the following numbers during DECELERATION; my foot off the gas pedal: 1. STFT for B1 and B2 = 0 2. LTFT for B1 and B2 = 0 3. A/F ratio sensor 1 bank1 = 3.7 to 4mA (before CAT bank1) 4. Eqv ratio 11 = 1.99 5. A/F ratio sensor 1 bank2 = 3.5 to 4mA (before CAT bank2) 6. Eqv ratio 12 = 1.98 7. O2S2B1 = 0V (after CAT bank1) 8. O2S2B2 = 0V (after CAT bank2) 9. RPM is about 2000 10. MAP = 5.9 inHg Then about 1/2 hour driving (acceleration and deceleration), I then park the car and let it IDLE: 1. STFT bank1 = -11.7 2. LTFT bank1 = 0 3. STFT bank2 = 0 4. LTFT bank2 = 0 5. A/F ratio sensor 1 bank1 = -0.007 to -0.0085mA (before CAT bank1) 6. Eqv ratio 11 = 0.999 to 1 7. A/F ratio sensor 1 bank2 = -0.019 to -0.070mA (before CAT bank2) 8. O2S2B1 = 0.76V (after CAT bank1) 9. O2S2B2 = 0.2 to 0.7V (after CAT bank2) 10. MAP = 9.7 inHg 11. Spark advance = 8 to 10 12. ECT = 201 13. Load = 32.5 14. RPM = 790 15. IAT = 134 16. TPS = 12.9 Then I turn off the engine and wait for about 10 minutes and start the car again at IDLE: 1. STFT bank1 = -25.7 2. LTFT bank1 = 0 3. STFT bank2 = 0 4. LTFT bank2 = 0 5. A/F ratio sensor 1 bank1 = -0.007mA (before CAT bank1) 6. Eqv ratio 11 = 0.999 7. A/F ratio sensor 1 bank2 = -0.050mA (before CAT bank2) 8. O2S2B1 = 0.6 to 0.825V (after CAT bank1) 9. 02S2B2 = 0.2 to 0.7V (after CAT bank2) 10. MAP = 9.7 inHg 11. TPS = 13.3 12. IAT = 147 13. ECT = 207 Questions: 1. Seems like when I drive, the numbers are getting better until at idle STFT bank1 go back to -25.7? Does 02S2B1 (after CAT bank1) is normal? the 2 A/F ratio sensors (pre CAT) are ok? 2. One more thing, yesterday, at IDLE, I rev the engine to 3000 RPM for sometimes, the P0172 (System too rich bank1) and P0430 (CAT system efficiency below threshold bank2) came up with Engine Light on. I was surprise P0430 for bank2 CAT. Should it be for bank1 instead? Yes, the O2S2B2 values indicated potential CAT problem for bank2, should it be CAT for bank1 instead because of bank1 STFT is too negative - 25.7? 3. Can a restricted CAT bank2 causes STFT bank1 negative? 4. Or the other way around, can STFT bank1 is too negative which can cause restricted CAT bank2 problem? 5. I just order a "smoke machine" which will arrive in next couple of days, will do smoke test in all system. Any other test(s) should I try? Please advice. Many thanks & God bless, Hai

I'm confused, in your LT ST basic overview you where saying you can Identify the part causing the problems with LT and ST, be it injectors or pump. But in this video your saying it more or less is worthless for detecting a misfire. Are you saying that if you can get to the point where you know it's lean then LT and ST can be useful? Can this be used for cars with no schrader on the rail and are a pain to test fuel pressure?

Hey scannerdanner just wanted to talk to you about a test like this using the map sensor and O2 sensors data to find a type of cylinder miss. you might already know about this and if so just let me know. its pretty much what your doing but little different. so here we go first to determine if its a possible mechanical miss look at the map sensor data. were using the map sensor as a vaccum guage. so if the map looks smooth not jumping around like you see on vaccum guage with mechanical problem then look at o2 sensors. upstream and down stream. mainly downstream is what will help lead you in right direction to a fuel issue or ignition issue. now lets say downstream o2 is reading lean this will lead you to a fuel issue because the cat is letting go of this extra o2 that doesn't need to store since the injector isn't supplying fuel. now vise versa if the downstream is reading rich now were looking at an ignition miss because now the cat is using all the oxygen to burn off all the extra fuel. now its a great test I was taught by a very smart instructor like yourself and wanted to share with you if you didn't know about this. if so just let me know. if you know about this and or have video of this I would like to know. thanks and keep the great vids coming.

Nice video bro

Hi Mr. Scanner? Could you kindly enlighten me on where the fuel trims feedback fetch from? is it from the oxygen sensors? I recently had a scenario where P0200 was stored in a Toyota Rav4 with 2AZ fse engine and vehicle wouldn't rev up and it will stall but by disconnecting the oxygen sensors it would behave well. This is where i noticed that even with the oxygen sensors disconnected the fuel trims were reading rich! so my quetion is other than oxygen sensors is there anywhere else the ECU reads fuel trims from? Thanks.

Iam just got out of school and iam buying more tools. I have all the regular sizes in metric and sae. Should I get the jumbo or large size sockets for SAE? I know metric is used more now like on axle bolts. Would It be better to just do the larger metric sizes for now?

What about using pre and post cat O2 sensor data? I've read a bit on that and ( even though I haven't personally tried it yet ) it seems like a very good way of pinpointing what type of misfire you've got.

Excellent.

And I need to improve my knowledge ...thanks

What is the misfire was from a stuck open injector?

- Scanner: Observe ST TRIM & LT TRIM o Ignition misfire event: x Most of the air/fuel mixture goes into the exhaust x Some fuel condenses in the cylinder and enters the oil → initial upstream O2S lean reading. STFT rises up to 15%. x Catalytic converter tries to burn the unburned HC, using up all the O2 from the bad cylinder → downstream O2S is 900mV.

hey bro....i do have a problem with my toyota corolla....everytime i slow down... and applying my gas on it is not doing what he is suppose to do.....it takes time for it to accelerate....

sorry, I will make sure to not do that next time. that is good advice

Im having trouble with a customers car. Its a 1.2 clio 16valve 2003. Stft +20 ltft +20 more or less and its misfiring in lower rev range pretty badly. I replaced plugs and coil pack no difference, oddly enough i put my hand over the air intake to monitor difference in stft and ltft but theres no suction ???. I did a smoke test and found no leaks so now im more confused how is it getting enough air to stay running.

What would cause a normal stft +/-3 but long term to be +20 or higher?

Wrong brand spark plugs will cause a missfire,I see this a lot with Bosch spark plugs installed a lot and seen this with E3s once so far.Ford tritons with the COP ignition system are known for going through coils.Had one Ford in for a missfire showing no codes,had the customer take it to the dealer and it was a bad ECM.Everything else looked good.GM 4.8s,5.3s and 6.0s,spray down the intake gasket area when a missfire code appears.If changes,the intake gaskets are bad,blow out taking in air

I have no codes or misfires that show but that cylinder events test says fail, what does it mean?

Hands down, best channel of it"s kind. I"ve got a P0300 DTC, along with flashing check engine light, that is intermittent.Shortly after the event occured, st fuel trim went up to 25%, and then got to 0%, and the voltage of both (upstream and downsream) o2 sensors went to about 0,03v, and 0,2v respectively. But because the fault was intermittent, as soon as the engine management light turned off, the upstream o2 sensor voltage would start oscilate normally, and the voltage of the downstream o2 went back to about 0,7v.(narrow band, as far as I know). What I can"t understand is why the stft stays at 0%, even when the upstream sensor"s voltage starts to oscilate normally? Allthough at first i suspected the sensor, then i thought that it had to go into open loop, and that was likely why the stft went to 0%. But I still don"t know what to make of the low voltage on both sensors, when the check engine light is blinking, followed by the normality of the voltage value on both sensors, when the check engine light turns off, becouse at the same time the stft remains at 0% at this later moment. So, if I was to say that the voltage comes back becouse it briefly reenters into closed loop, shouldn"t then the stft start to show a value other than 0%? Your video guids me towards the conclussion that tere is a greater lean condition than i thought that forces the emcu to go in open loop, thus one or even more injectors at fault(given the p0300). I know it"s not your job to answer this kind of questions, but I desperatly seek any help i can get in order to try to fix it on my own, given that for the moment it is not safe to wonder arround autoshops, with the pandemic still on a crescent trend in my country.

subaru impreza wax DTC P0171, RPM 740 , STFT 25 and LTFT 14.8 , MAP36.0 . MAF 2.1 . g/s . oxygen sensor 3.943 V if engine RPM 3000 STFT 2.3 LTFT 14.8 what is causing it ? thank you Danner ,

Honda accord V6 3.0L: Misfires all cylinders sometimes, but STFT = -25% on bank1 ONLY. New spark plugs, new injectors, new PCV, new MAP, new gaskets (intakes, throttle body, EGR, top cover engine), clean throttle body, clean EGR, did valves adjustments, test evap purge valve, new fuel regulator, test coils, very good compression in all cylinders. Still get -25% STFT on bank1 ONLY and sometimes Misfires all cylinders. Any suggestions? ScannerDanner 15 minutes ago Okay, that rules out a restricted cat. Next step would be an injector balance test for me. What year is the honda? Some models will use downstream O2 sensors for fuel trim control, but that is usually accompanied with a wide band AF sensor upstream. If it has downstream fuel trim, we need to focus on the B1S2 and B2S2 sensors. Of course we need to also focus on the upstream O2 sensors as well. (B1S1 and B2S1). In fact after typing this, that is where I would start. (Testing the upstream O2 O2 sensors) Hai Ho 1 second ago @ScannerDanner Good morning ScannerDanner. I thank you for your great inputs. It is 2004 Honda accord V6 with 200K miles. The 2 pre-cat sensors are A/F ratio sensors and I did swap them around and they both are ok; one is new from last year replacement (Amp numbers with equiv. ratio is 0.99 or 1). At idle, After the CAT sensors: bank1 S2 stays high around 0.85 and bank2 S2 does fluctuates from 0.2 to 0.7 and there are times around 0.6V for quite sometimes. I'm questioning this B2S2 for the bank2 CAT however STFT is -25% for bank 1 ONLY. Strange? What is injector balance test? Talking about "balance", the compression numbers for bank1 are 150, 145, 150 and bank2 are 180, 175, 180. Any thoughts?

What would cause a negative short term fuel trim with cylinder misfire? Fuel leaking into crankcase/PCV system (high pressure fuel pump seal bad)? Vacuum leak? Bad sensor? Poor compression? Carbon build up on intake (GDI engine). The engine also has run-on after shut off. 2013 Equinox 2.4L