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Old 05-11-2013, 12:29 PM #1
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P0171 Lean bank 1 Question

i have a 1999 4runner 3.4 4x4 automatic with 150k miles. my question is about this code p0171. it started 6 months ago, with the check engine light coming on and going off without being reset. just recently the light has came on , and is staying on. symptoms it has now is smell of fuel (mostly around tailpipe) and significant drop in mpg ( from 17/20 to 5/8 ) . I have cleaned the maf, replaced the maf with new, replaced fuel filter , replaced tps , fixed exhaust leaks, checked for vacuum leaks. currently i'm thinking about the upstream o2 sensor(because its cali emisions its refered to as a a/f sensor ) . it runs fine, idles normaly , no power loss , no hesitation . any help would be great.
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Old 05-11-2013, 12:47 PM #2
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I don't mean to thread-jack here but I don't want to start a new one either, on the same day for the same issue. My 3rd gen just threw this same code yesterday. It was driven on the highway for about 4 hours after getting the code and there is no noticeable change in mileage, the way it runs, no funny smells, etc. From the searching I've done, it seems like there are a variety of things that people have done that finally fixed the problem. Does anyone have a list of things to do in order of likelihood? Maybe starting with running something through a tank of gas to taking things apart and cleaning/replacing?
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Old 05-11-2013, 02:27 PM #3
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Below is a straight copy/paste of diagnostic procedures in fairly basic terms with limited tools. From Identifix/IATN. The numbers next to different items at the bottom show the number of technicians reporting to have fixed this code with each of the following parts. The vast majority (including my own vehicle) of these I've seen are simply due to a dirty MAF sensor and can be easily resolved with cleaning. I have seen many many people clean them incorrectly thinking mistaking the intake air temp posts for the MAF heater wires. What brand MAF sensor did you replace with... wouldn't be the first time I've seen the code return due to an inferior part. Also on you emissions tag under the hood is it listed as Federal or California Emissions? If it is a California vehicle its a bit of a different approach. Let me know if you need anything or have any further questions and we'll try to get it worked out.


1999 Toyota 4Runner 3.4L, Eng Cfg V6, Eng Des 5VZFE, USA/Canada

Hotline Archive
With OEM Direct diagrams, components, TSBs, and R&R procedures

Number: 482996
Vehicle Application:
1998 4Runner 3.4 1999 Tacoma 3.4
1999 4Runner 3.4 2000 Tacoma 3.4
2000 4Runner 3.4 2000 Tundra 3.4

Customer Concern:
Has a check engine light on and a code P0171.

Tests/Procedures:
1. Monitor the fuel trim readings at idle and at 55 MPH driving down the road. If the numbers are low at idle but climb on a part throttle cruise, the airflow meter could be reading low or it may also have low fuel pressure. The mass air flow sensor grams per second should be 3.3-4.7 at idle and 12.9-18.3 at 2500 RPMs no load, if the readings are low the sensor could be bad.

2. Monitor the calculated load reading on a wide-open throttle acceleration. The reading should go to 85% or more. If not, then the airflow meter could be faulty.

3. Monitor the oxygen sensor inputs, both front and rear oxygen sensor signals. The oxygen sensor should not stay high or low voltage for extended periods of time. If the oxygen sensor shows low voltage and the fuel trim readings climb, check the performance of the oxygen sensor.

4. Check the vehicle for a vacuum leak by spraying carburetor spray around the intake manifold while watching the fuel trim readings. If the readings change, there is a vacuum leak.
Tech Tips:
If fuel trim readings are high at idle and go down on a free rev, there is a vacuum leak. If the fuel trim readings are high at idle and at cruise, suspect fuel injectors.


Diagnostic Codes
P0171

Confirmed Fix
225 - Mass Air Flow (MAF) Sensor
9 - Engine Air Filter, Mass Air Flow (MAF) Sensor
4 - Oxygen (O2) Sensor(s)
2 - Fuel Pump
2 - Heated Oxygen Sensor (HO2S) Bank 1 Sensor 2 (B1S2)
2 - Heated Oxygen Sensor (HO2S) Bank 1 Sensor 1 (B1S1)
1 - Front Oxygen (O2) Sensor
1 - Vacuum Leak
1 - Fuel Injector(s), Mass Air Flow (MAF) Sensor
1 - Cruise Control Vacuum Line
1 - Intake Manifold, Mass Air Flow (MAF) Sensor
1 - Mass Air Flow (MAF) Sensor, Throttle Body Cleaning Procedure
1 - Mass Air Flow (MAF) Sensor, Mass Air Flow (MAF) Sensor O-Ring
1 - Exhaust Pipe, Mass Air Flow (MAF) Sensor
1 - Air Intake Tube
1 - Mass Air Flow (MAF) Sensor, Throttle Body
1 - Air Fuel Ratio (AFR) Sensor
1 - Engine Vacuum
1 - Oxygen (O2) Sensor, Bank 1 Sensor 2
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Last edited by EatonKyleH; 05-11-2013 at 02:37 PM.
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Old 05-11-2013, 02:32 PM #4
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Quote:
Originally Posted by jhueter View Post
i have a 1999 4runner 3.4 4x4 automatic with 150k miles. my question is about this code p0171. it started 6 months ago, with the check engine light coming on and going off without being reset. just recently the light has came on , and is staying on. symptoms it has now is smell of fuel (mostly around tailpipe) and significant drop in mpg ( from 17/20 to 5/8 ) . I have cleaned the maf, replaced the maf with new, replaced fuel filter , replaced tps , fixed exhaust leaks, checked for vacuum leaks. currently i'm thinking about the upstream o2 sensor(because its cali emisions its refered to as a a/f sensor ) . it runs fine, idles normaly , no power loss , no hesitation . any help would be great.

AHH FORGET MY LAST POST! although the T4R Finally could use it.

You did state it was a California Emissions Vehicle. The Air/Fuel ratio sensor is the common cause of this code/condition.

SO HERE YA GO!

Vehicle Application:
1999 4Runner 3.4 2000 Tacoma 3.4

Customer Concern:
Check engine light is ON. Lean code P0171, but the engine is running very rich. California emissions.


Tests/Procedures:

1. Verify battery voltage to the A/F ratio sensor heater on the White/Red wire on the harness side with the key ON.

2. Unplug the A/F ratio sensor and turn the key on and verify that the Red wire and one of the White wires has about 3.0 and 3.3 volt reference from the Engine Control Module (ECM) with the key ON.
Tech Tips:
The California emissions vehicle uses an Air/Fuel (A/F) ratio sensor in place of the front oxygen sensor for mixture feedback information. The ECM thinks that the engine is running lean and keeps adding fuel, yet the engine is rich, so the ECM is getting incorrect information from the A/F ratio sensor.

Diagnostic Codes
P0171
Potential Causes

4 - Air Fuel Ratio (AFR) Sensor
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Last edited by EatonKyleH; 05-11-2013 at 02:44 PM.
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Old 05-11-2013, 02:46 PM #5
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I had this code for like a month and only recently finally got it fixed. For me it was the MAF that was dirty. There are two pairs of wires that need to be cleaned. The second pair that almost everybody misses is opposite of the visible. It sits inside the slot.

My other suggestions are: vacuum leaks (I replaced all the ones that were cracked) and/or O2 sensors (I replaced both) or if its really dead a new MAF.

Remember to only clean with MAF sensor cleaner.
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Old 05-11-2013, 05:08 PM #6
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I am a little confused now. Mine is also a Cali emissions and the upstream and downstream o2 sensors have been replaced. The front one was done at a dealer prior to me buying it and it was in response to a complaint about a cel. The downstream was replaced by me right after buying it as a result if a p0440 (I think). I used a denso and it did fix the problem. So, with a Cali, is the issue most likely the MAF or the AF? And is the AF one in the same as the upstream o2?
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Old 05-11-2013, 06:12 PM #7
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Quote:
Originally Posted by T4R_Finally View Post
I am a little confused now. Mine is also a Cali emissions and the upstream and downstream o2 sensors have been replaced. The front one was done at a dealer prior to me buying it and it was in response to a complaint about a cel. The downstream was replaced by me right after buying it as a result if a p0440 (I think). I used a denso and it did fix the problem. So, with a Cali, is the issue most likely the MAF or the AF? And is the AF one in the same as the upstream o2?
Yes with California emissions it is likely to be caused by the air fuel ratio sensor as the federal uses a standard o2 sensor. It's the forward sensor. However it is still likely to be caused by a dirty Maf with both and since you are not noticing fuel economy decline or performance issues I would start with cleaning the mass air flow and clearing codes.
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Old 05-11-2013, 06:20 PM #8
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P0171

From experience having just dealt with a P0171 I can tell you it probably isn't the O2 sensors as that would throw another code. I solved this problem with a thorough MAF cleaning (check other threads with pictures as I didn't know about one of the wire locations and didn't clean them initially), throttle body cleaning (without removing) and replacing several cracked vacuum hoses. Don't just visually inspect the vacuum lines. Twist them and manipulate them a little to make sure they aren't cracked or split. Soon as I did all this the P0171 was gone.
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Old 05-13-2013, 12:16 PM #9
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Just finished cleaning the MAF. Nothing was caked but everything was pretty black. All nice and shiny now using CRC MAF Cleaner. I cleared the code with my scanner. We'll see if it comes back.

So, is the jury split on whether or not it could also be the upstream O2 sensor as well on a Cali Vehicle? I know, when I went under mine to replace the downstream sensor, I could tell that the upstream one had been changed because it looked too new compared to everything else. When I registered on the Toyota owner's website and looked at the service history, in 2008, the AF sensor was replaced as a result of a P1135, P1130, and P1125. Then, in 2009, a service was done to clean the MAF because of a P0171 and the notes mention that it was covered in grit. I found no grit this time but it was dirty.
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Old 04-11-2014, 08:49 PM #10
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Im facing this as well... already cleaned the MAF, and havent noticed any horrible drop in MPG (city is normally bad). I suppose I could replace the hoses... there is definitely some cracking in mine. Does anyone recommend an "air hose kit" or where or what exactly to replace? Dealer parts? Advance Auto? I suppose I'd just replace all the rubber from the intake to the manifold.
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Old 04-14-2014, 10:24 AM #11
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Forgot to mention... Im running a K&N filter... which Ive been reading is not a good option for the 3.4. The oiled filters do not agree with the MAF and can dirty it up pretty quickly. Pulling that out and going back to (factory spec) paper; giving the MAF another cleaning. We'll see.
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Old 04-14-2014, 01:54 PM #12
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Quote:
Originally Posted by denuz View Post
Forgot to mention... Im running a K&N filter... which Ive been reading is not a good option for the 3.4. The oiled filters do not agree with the MAF and can dirty it up pretty quickly. Pulling that out and going back to (factory spec) paper; giving the MAF another cleaning. We'll see.
Are you running the full intake or just the drop in for the factory box? I had a K&N full intake and it plagued me with the P0171 codes for ever!! I swapped back to my factory intake and have not seen the code since(2-3 months now). Also, I talked to the service manager at my local toyota dealer and he verified that the 5vz-FE 3.4l doesn't like having a full intake, recognizes the extra air and triggers a lean code. Just thought if throw my experience out there.
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Old 04-14-2014, 02:43 PM #13
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Just running the drop in filter... but it seems to be enough to piss off the MAF sensor. There were no issues beforehand. I dont suspect there's an air leak anywhere... Im also getting 17-23mpg, so things are working well.
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Old 04-08-2015, 09:00 PM #14
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Proper cleaning of MAF

Hi guys,I've been all over the web forums looking for a solution to my problem. I wanted to say thanks for all the advice. I was pulling the P0171 code too, and struggled to get the code to clear. After a lot of anguish and terrible milage, I read somewhere that I was cleaning the MAF wrong. (joeflem)

THERE ARE TWO WIRES DOWN INSIDE THE MAF THAT YOU CAN'T SEE. The exposed and obvious part on the MAF was clean, but for 35,000 miles I hadn't cleaned it correctly. I know it has been said before, but I just wanted to point this out in case someone like myself may have overlooked it. Look inside the MAF, deep inside the opening. May be a silly thing for those who are more experienced, but it solved my problem and I'm thankful for the community.
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Old 04-08-2015, 09:38 PM #15
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Replace upstream A/F sensor since it probably needs it anyway as well as check you intake plenum gaskets. I was battling this code for a number of weeks. Ran a smoke test and saw smoke coming from the plenum. Replaced both gaskets and code went off and has not come back on
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