Sensor 2 is strictly cat monitoring. If the cat is doing it's job it is normal to stay in the middle 450mv-600mv. Some cars 400-700 is fine. As i said though without a scope you are not seeing true live data because 02 sensors move too fast. Also until it is fully warmed up it is normal for the rear 02 sensor to be erratic.

I have a lamda (a/f) sensor as my sensor 1 instead of a wideband 02 on my 07 Corolla. I hate reading those lol.

 

STFT is a very quick pid that lets the ECU know if instant adjustments are needed. Its normal for it to go +/- like 15%. LTFT is the more important pid letting the ECU know its having a hard time keeping the proper a/f ratio - too lean/rich. Above 20% +/- is getting high.

 

You can do an event log in torque. Just hit start and view later. Safer than monitor and drive at the same time.

 

Also check vacuum hose routing, 03 Vibe had the one at the bottom of the air box mixed up with the emissions vacuum at the rear of the air box.

 

I had the P0171 on a maxima years back. The P0171 was the only code. I went through a boat load of diagnostics and replacing parts (plugs, O2 sensors, etc). In the end, it turned out to be a MAF sensor reading low (not low enough to throw a MAF code, but low enough to cause a lean mixture and throw the P0171 code).
The MAF was reading right at the low end of the specified range, but still in range, so I initially dismissed this as the culprit. It was only after I compared my MAF readings to another Nissan that I replaced my MAF and voila, issue solved.

I’ve never taken readings from my Camry, but I could let you know what the MAF at idle is for my Camry.

I’m not saying it’s your MAF for sure, but it’s something to think about.

check out these threads. It’s Nissan, but similar. This was really a pain in the rear end as you can read.

P0171 - How do I fix this?? - Maxima Forums

5th Generation Maxima (2000-2003) - P0171 - How do I fix this?? - P0171 - Fuel Injection System Too Lean (Bank 1) the light came on at 90K miles.

maxima.org

P0171 - But everything tests out OK... Having trouble diagnosing - Nissan Forum | Nissan Forums

forums.nicoclub.com

As you’ll read, the code stayed away for a bunch of years after MAF replacement. When it came back the 2nd time, it’s was a 10 minute diagnosis and repair. Much easier when you know what to look for.

One final note, if you do end up trying the MAF replacement, be sure to shop around. As you’ll read in my posts, the local dealer wanted $400 for the sensor I was able to buy an OEM Nissan sensor online from a dealer in TX for under $100.

Also, when you replace the MAF, it takes a while for the LTFT to settle back toward zero. So you might set another code while they settle. With the Nissan, if you ran briefly with the MAF disconnected, the ECM would reset the LTFT to zero (a nice backdoor way to instantly reset them w/o having the dealership computer system). You’d get a MAF code when doing that, but you could easily clear that with the obd ii. Not sure if Toyota is the same, but it’s worth a shot. If you can’t reset the LTFT, just keep an eye on them via OBS II and see if they drift back closer to zero.

 

1. P0171 was a very common code for these cars back in the day at the dealership . I see you went front to back on this ordeal. Just like avritch was saying i bet the mass airflow sensor is out of range causing your p0171 .How is the throttle response when you blip the throttle in park? You can get the mass airflow spec and compare them to your scan tool reading. You can remove sensor only 2 phillip screws and clean it with some brake cleaner or mass air cleaner then reassemble and check readings to confim repair.

 

If you have the time, this you tube video may shed some light on it. Good luck.

 

I had a Corolla I was working on one time that had bad injectors (they were clogged) tested them with a tool designed to pulse each injector individually while monitoring fuel pressure drop. If your front O2 sensor is good and your MAF sensor is good (avoid any type of oiled performance air filter) and you don’t have any vacuum leaks, AND you closely inspected the air snorkel going from the air cleaner assembly to the intake for tears between the ribs in the rubber. I would then, and only then suggest new injectors. As an aside a smoke tester is the best method to check for vacuum leaks. A quick and dirty way of testing it is too warm the engine then spray the entire intake area with brake clean and listen for a change in how the engine runs. I’m an ASE certified master tech btw. Hope this helps. For what it’s worth I am highly suspect of either a vacuum leak or a tear in the intake air boot. What is the year of the car, and what are the miles on it. Also what state has the vehicle lived in?

 

Thanks. I did a propane test and a water leak test as well and I did not notice any change in RPM. I watched some youtube videos and got some ideas about a smoke machine. I'll build that tomorrow and try to test for any vacuum leaks tomorrow. It is a 03 corolla and has around 220k miles and the car has lived all its years in NC (with some salt on the road in winter).

 

Thanks a lot for all the excellent advice. I just replaced my spark plugs today with NGK Laser Iridium and nothing changed to be honest. I am still getting high fuel trims. I noticed that when the car is moving, the short term fuel trim oscillates around zero. However, when the car is geared in Drive and I am stopped, the short term fuel trim goes to around +20%. I've attached a log of a short drive to this comment.

When in drive but stopped, my specs are as follows:
Speed (mi/hr) 0.0
G(calibrated) 0.0085
Engine RPM(rpm) 750
Fuel flow rate/minute(gal/min) 0.0030
Fuel Trim Bank 1 Long Term(%) 25.0
Fuel trim bank 1 sensor 1(%) 18.0
Fuel Trim Bank 1 Short Term(%) 17.97
Mass Air Flow Rate(g/s) 1.89
O2 Volts Bank 1 sensor 1(V) 0.04
O2 Volts Bank 1 sensor 2(V) 0.04
Throttle Position(Manifold)(%) 11.4
Timing Advance(°) 15.0
Turbo Boost & Vacuum Gauge(psi) -12.47

I checked some of the default values and I suspect that both my MAF and oxygen sensor are going bad. My MAF sensor values are within spec at idle but shows 10 at 2500rpm and it should be below 8gm/s which suggests that it is out of spec but not that much. However, the oxygen sensor should be between 0.1 and 0.9V but at idle it is 0.04.

Can someone more knowledgeable please check my log and let me know?

Moreover, do both oxygen sensors fail in tandem?

Happy New Year!

Let's hope that 2020 is a good year for all our toyotas!

 

Thanks a lot for all the excellent advice. I just replaced my spark plugs today with NGK Laser Iridium and nothing changed to be honest. I am still getting high fuel trims. I noticed that when the car is moving, the short term fuel trim oscillates around zero. However, when the car is geared in Drive and I am stopped, the short term fuel trim goes to around +20%. I've attached a log of a short drive to this comment.

When in drive but stopped, my specs are as follows:
Speed (mi/hr) 0.0
G(calibrated) 0.0085
Engine RPM(rpm) 750
Fuel flow rate/minute(gal/min) 0.0030
Fuel Trim Bank 1 Long Term(%) 25.0
Fuel trim bank 1 sensor 1(%) 18.0
Fuel Trim Bank 1 Short Term(%) 17.97
Mass Air Flow Rate(g/s) 1.89
O2 Volts Bank 1 sensor 1(V) 0.04
O2 Volts Bank 1 sensor 2(V) 0.04
Throttle Position(Manifold)(%) 11.4
Timing Advance(°) 15.0
Turbo Boost & Vacuum Gauge(psi) -12.47

I checked some of the default values and I suspect that both my MAF and oxygen sensor are going bad. My MAF sensor values are within spec at idle but shows 10 at 2500rpm and it should be below 8gm/s which suggests that it is out of spec but not that much. However, the oxygen sensor should be between 0.1 and 0.9V but at idle it is 0.04.

Can someone more knowledgeable please check my log and let me know?

Moreover, do both oxygen sensors fail in tandem?

Happy New Year!

Let's hope that 2020 is a good year for all our toyotas!

Typically, you should replace in tandem. The other one isn't too far behind to fail.

Have you tried cleaning the maf with maf cleaner?

 

How was it running before doing any work to it? Was it just sluggish or wouldn’t start? I recently had to replace a coil pack because it was running sluggishly, disconnected each plug to listen for spark while running, no spark on cylinder 2 or 3, I forget too lol, but ya, pulled it out and there was oil on the end of the coil over and atop the plug from I’m guessing previous owner having work done and a valve cover bolt getting left out.

 

I did not notice any difference before and after CEL came on. I did the ignition coil test by removing each connector and the RPM changed drastically and then came back to normal after 1-2s. I made a homebuilt injector tester yesterday to check all my spark plugs and injector coil and they all seem to be working properly. I'll write some more about that once I fix that lean code. I'm planning to build a smoke machine tomorrow and test it on Thursday or Friday just to make sure that all my bases are covered.

 

I would leave it for now.

Can you post the screen shot of how the O2 oscillated earlier?

 

Rather than wasting time and money throwing parts at a car, why not bring it to a quality shop for a proper diagnosis and repair? A P0171 can be caused by various factors. Without having the vehicle in front of me I can only offer a guess.BTW, the Bluetooth scantool you're using is crap especially when compared to a Techstream or techstream lite. Even a quality generic tool is a MAJOR upgrade. Nothing is a substitute for expertise, no matter what you've been told. Regardless, good luck.

 

I totally agree with you and agree that a quality shop will probably diagnose the issue much faster than me.

However, I learned a lot from this P0171 code. For example, it was a good learning experience in understanding how a modern car operates from the fuel pump to the ignition and why the system is under vacuum. It might be trivial since you are a master mechanic but I never got the opportunity to learn how a car operates.

I also got the opportunity to clean the fuel injectors, do a smoke test, check my spark plugs and ignition coils and clean the throttle body.

 

I think it quality technician will find this problem within 2 hours. I honestly think throwing parts at it is unadvisable. If it does need a MAF sensor, you could likely get a used one for $25 to $40 from a local salvage yard.

 

Ok.... trying to help with real data..
This data is from a 2012 Toyota Camry with the V6.
The data is with the engine warmed up sitting at idle.
Outside air temp = 37degF

MAF = 3.6 g/s @700 RPM
LTFT = ~10% on both banks (see attached photos)

I took a video of the MAF reading at various RPMs (racing the engine while in park), but the file type is not allowed to be posted, so here is the data

RPM=700 MAF = 3.6 g/s
RPM=1500 MAF = 7 g/s
RPM=2200 MAF = 10.5 g/s
RPM=3000 MAF = 13.5 g/s

As I said earlier, the 1.8g/s you're seeing at idle seems low. If you're also seeing low reading at higher RPMs vs the data above, then you can be pretty confident you've got a tired/lazy MAF sensor.. Looks like Toyota uses Denso for their OEM and those are available for apx $65 to $85 (similar to what I paid for the Nissan OEM part). I would recommend against a used part based on my prior experience (i.e. MAF sensors do get "lazy" over time and read low... and cleaning them didn't work for me in two cases)...

We typically don't think of the MAF as a scheduled maintenance item needing replacement on a defined interval, however, as you'll read from various automotive experts, the MAF sensors do wear over time. My Nissan lasted apx 80K miles & 5 years for each sensor.

My Toyota is apx 7 years old now, so I know I'm on borrowed time...

If there are any other readings you would like me to take, please don't hesitate to ask

Regards and good luck

 

My Toyota is apx 7 years old now, so I know I'm on borrowed time...

Dont think such pessimism is warranted for Denso MAFs. My corolla's MAF sensor is over 15 years old and still the original. A lot of members on this sub-forum also have not replaced MAF sensor. Some do go bad.

YMMV

 

hevster1 had some good advise for you. You may get lucky by being a parts cannon but you'll more likely empty your pockets first.
Did your car stall out or labor when idling, did it hesitate when you try to rev it up or pull away quickly from an idle? Your idle is high by a hundred, you have very low MAF reading for idle, should be about 3, your fuel trims are way out of norm, your vacuum is abnormal, fuel flow should be about double at idle. I'm surprised that it's running at all in any usable fashion. Anyway, changing coils, plugs, MAFS, O2's is not going to help those readings.
I suspect you have a vacuum leak primarily. There should be a diagram of the vacuum lines somewhere under the hood. Look CAREFULLY and METHODICALLY from air cleaner box to the throttle body at all the vacuum lines, that they are attached and none are cracked. Spray a fine mist of water around there while it's idling to see if you get a change in idle. Grab, pull and shake things as you're doing this, while being careful not to get your tie caught in rotating machinery. If nothing happens then it could be either/or both the charcoal canister or the brake booster. If you pull the line off the brake booster and plug it what happens to the trim readings? If you have a vacuum pump check the canister that it holds vacuum.

 

Did you check the PCV lines and check valve? Did you clean the inside of the throttle body, any bypass valves and lines? First, assume all of the sensors are working properly. Don't just change parts because they're not telling you the story you want to hear. Don't shoot the messengers, you have to listen to all the stories(sensor readings) to figure out the problem. All the sensors are telling you you have a vacuum leak.

 

A simple smoke test will tell if there is a major vacuum leak. If you don’t have the equipment to do so any quality local shop does as a smoke machine is a basic necessity in diagnostics with today’s vehicles.

 

A simple smoke test will tell if there is a major vacuum leak. If you don’t have the equipment to do so any quality local shop does as a smoke machine is a basic necessity in diagnostics with today’s vehicles. These readings do indicate a massive vacuum leak.

 

Smoke tests are great and have their place but if the diaphragm in the brake booster is leaking it may not show and it's way easier and quicker to pull the hose off of it, plug it and watch the reaction on the OBD reader. This goes for the evap canister as well.
Dirty MAF's do not give you low vacuum readings. However, FIRST, I would clean the MAF, easy, and then the throttle body next, harder, but I repeat, a MAF, dirty injectors, bad coil and/or plugs ARE NOT GOING TO GIVE YOU LOW VACUUM READINGS.
Please everyone check this video out, very informative and a good one to review if you find yourself with a P0171 code again. It's a bit long but it's so worth it.

One other thing I overlooked is a compression test. An eng with this many miles may have valve problems which may also be the cause of your misfires and associated low vacuum. Hate to be the bearer of bad news but it may be plum wore out, not worth fixing.

 

Thanks a lot. I watched the video very closely and the MAF sensor was the culprit. It was faulty and was reading on the low side which was causing the car's computer to malfunction. No more rough idle and no vibration once the MAF sensor was replaced! Thanks for sharing the video. It helped a lot in diagnosing the sensor issue.
Appreciate it.

 

Any update op?

Side note. Early this morning, I was monitoring my LTFT, STFT, and both O2 sensors. What I noticed was that LTFT remained flat and timing was advanced at 32 degrees. Isn't LTFT supposed to oscillate more frequently compared to STFT? Also, why would the timing be advanced at 32 degrees at 4000 rpm?

 

Long term fuel trims move much more slowly than short term fuel trims.

 

Why is that?

 

The amount of fuel required to keep the fuel air mixture in range will vary based on various engine parameters. So the computer will slowly adjust the long term fuel trim to find the equilibrium point where you get the proper fuel air mixture at stable operating points. However, there are instantaneous changes in engine operation that require immediate adjustments of the fuel air mixture (like acceleration). The computer adjusts to those instantaneous needs by adjusting the short term fuel trim. You can think of the long term fuel trims as a long term average of the short term fuel trims. Or in Engineering lingo, the long term fuel trim is the integral portion of a PI control loop and the short term trim is the proportional component.

 

Update:
Thanks a lot for all your expert advice! I really appreciate it. I did not bring the car to a expert mechanic because of the cost associated. Yes he will find the problem faster and I do not disagree with that.

I made a DIY smoke 'machine' (using mineral oil in a bottle attached to a coil and a hand-pump) to test for vacuum leaks and was surprised that there was none. I repeated the test just to be 200% sure. To check if the smoke machine was working, I opened the oil cap and smoke came out.

Based on 'avritch' numbers, I checked out the MAF sensor and to my surprise, it was reading on the lower side! I also did a resistance test (should be around 2.2kOhm at 70F) and checked the MAF numbers at idle (700RPM and 2500RPM) and both numbers were under the standard values. I am not sure what caused that and assumed that it was failing but consistent! I went and purchased a Denso MAF sensor and cleared the fault code. Once I installed it, my STFT was oscillating around zero. No codes from the 3 days of driving.

Thanks a lot for taking the time to respond! I genuinely appreciate that.

 

I'm so glad you got this figured out. This will be good to know for some others who have already replaced the intake gasket only to continure to have the P0171.

I also have a homemade smoke machine (using air compressor, a paint can with tubing, and burning incense). I'm curious if you could share your smoke device with us?

 

I also have a homemade smoke machine (using air compressor, a paint can with tubing, and burning incense). I'm curious if you could share your smoke device with us?

Chiming in here to perhaps save someone some time.

I was going to build a smoke machine, but thought I'd try this first as it only took me 5 minutes to hook up.
I ran air (no smoke) at 5 psi into the intake manifold via the brake booster line. Also capped off the line from the engine to the EVAP system since the test port cap said 1 psi only and I didn't want to damage it. Sealed the Throttle body by removing the air box lid and putting a wad of plastic bags in the intake hose. shoved a potato into the muffler outlet. Sprayed soapy water all over, was looking for leaks at the exhaust manifold too. Found a pretty good leak at the #2 injector. bubbled like crazy. No other leaks detected.

 

That's cool, but I also use it for evap (mostly for evap) and as you mentioned, it requires very low psi or else damage to the components (especially in the charcoal canister) can occur.

A potato! That's AWESOME! It's amazing all the different uses for potatoes: Boil, mash, fry, "stick 'em in a stew", roast, saute, ferment, etc. (I wouldn't be surprised if some people smoked potatoes). How 'bout posting a nice photo of that Spud o' Pipe?!

 

A potato! That's AWESOME! It's amazing all the different uses for potatoes: Boil, mash, fry, "stick 'em in a stew", roast, saute, ferment, etc. (I wouldn't be surprised if some people smoked potatoes). How 'bout posting a nice photo of that Spud o' Pipe?!

Alas, I did not take a photo, and the spud was destroyed digging it out of the tailpipe with a screwdriver. I did not come up with this, I read another thread where the guy said "plug the exhaust with a cork or potato". I did not have a big cork, but do keep a bag of spuds on hand in the kitchen. I remember years ago people would put taters in someone's tailpipe as a prank.