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Diagnosing bad MAF (P0171)

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Old 06-18-14, 03:36 PM   #1
vishesh
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Default Diagnosing bad MAF (P0171)

I have been scouring the web and posts here on CL but could not find an answer to what I thought would be a pretty straightforward question:

Is there an effective and accurate way to diagnose a bad MAF (Mass Air Flow) sensor?

I am getting a solid CEL with code P0171 (Bank 1 Too Lean). I have seen a lot of posts on P0171, both here and out on the web, and all suggest that a bad MAF is a likely culprit. For example, here are some posts on CL and the web:

Most of the advice says that replacing the MAF may resolve code P0171, but nobody explains how to first test whether the MAF is faulty -- and it would be nice to be able to diagnose a part as being faulty before replacing it (otherwise you're just taking expensive stabs in the dark). The only place I've seen that explains how to diagnose a bad MAF is http://repairpal.com/OBD-II-Code-P0171-and-P0174, where they say:

Quote:
There is a very effective "truth test" for any Mass Air Flow Sensor. Start the engine, let it idle, and then check the Barometric Pressure reading on the scan tool data. If the reading is about 26.5 Hg and you are close to sea level, you know that you have a defective Air Flow Meter because it is telling you that you are at about 4500 feet above sea level.
Problem is, I don't think the '99 RX300 has a separate barometric pressure sensor, so this approach won't work -- see these two posts:

So is there some other way to test whether a MAF is faulty? Can you take voltage measurements of some kind (i.e., with a multimeter) to test it? Can you diagnose a faulty MAF indirectly through the Long-Term or Short-Term Fuel Trim (LTFT / STFT)? Does anybody know?
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Last edited by vishesh; 06-18-14 at 03:39 PM.. Reason: Clarifying situation, editorial/stylistic changes.
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Old 06-18-14, 06:05 PM   #2
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First, before you go any further, understand that there are two parts to the MAF sensor. A thermocouple and a barometric type sensor. You may be able to measure the resistance across the thermocouple, but the barometric sensor will likely require an O-scope. In some MAF units, that barometric sensor may be a free wheeling fan that sends a voltage based on how fast it spins. This makes it difficult to test without special testing equipment.

I would ask you if you have done any recent work on your intake side of the vehicle and if you have, you may have forgotten to reconnect the MAF sensor, which would trip that CEL code.

If all is tight, review this thread about cleaning your MAF sensor: http://www.clublexus.com/forums/rx-f...-cleaning.html

Also, if you haven't done so, change your air filter and check for obstructions in the intake starting at the driver side headlight assembly where air goes into the intake.

Last edited by timmui; 06-18-14 at 06:10 PM..
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Old 06-18-14, 07:00 PM   #3
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Timmui, thanks for the input and information, I appreciate it. Do you have any specific information regarding the MAF used in the RX300 (which I think is the same used in many other Lexus and Toyota models)? I'd specifically be interested in diagnostic information. Also, are you sure the RX300 even has a barometric sensor? According to this ClubLexus post, it does not.
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Old 06-18-14, 07:42 PM   #4
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Quote:
Originally Posted by vishesh View Post
Timmui, thanks for the input and information, I appreciate it. Do you have any specific information regarding the MAF used in the RX300 (which I think is the same used in many other Lexus and Toyota models)? I'd specifically be interested in diagnostic information. Also, are you sure the RX300 even has a barometric sensor? According to this ClubLexus post, it does not.
I can't speak for what was written in that post. In the DIY, there is a picture that clearly shows the thermocouple (little brown glass bulb) and an opening which air blows up into. That is where the sensor is located. Is it a barometric sensor? I couldn't prove it short of disassembling my perfectly working MAF sensor.

However, judging from the design of the MAF unit, it is likely some form of pressure sensor. The hole is a pre-determined size and distance from the sensor in that shaft. If I were to design it, I would measure the air pressure being deflected into the shaft in relation to that circle to measure "wind speed" in that chamber. (it's a bit more complicated than that, but I don't want to bore you with the physics, since I am not willing to work it out precisely)

As for specific diagnostic information, that would be available in the service manual for the vehicle.

Last edited by timmui; 06-18-14 at 07:46 PM..
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Old 06-19-14, 01:03 AM   #5
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Quote:
Originally Posted by timmui View Post
However, judging from the design of the MAF unit, it is likely some form of pressure sensor
Wrong. This is a true MAF sensor not a MAP sensor which has to be referenced to barometric pressure. Do some Googling and you will quickly see how it works.
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Old 06-19-14, 01:20 AM   #6
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Quote:
Originally Posted by vishesh View Post
Timmui, thanks for the input and information, I appreciate it. Do you have any specific information regarding the MAF used in the RX300 (which I think is the same used in many other Lexus and Toyota models)? I'd specifically be interested in diagnostic information. Also, are you sure the RX300 even has a barometric sensor? According to this ClubLexus post, it does not.
Maybe this will help:

http://www.autoshop101.com/forms/h34.pdf
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Old 06-19-14, 07:19 AM   #7
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Originally Posted by artbuc View Post
Wrong. This is a true MAF sensor not a MAP sensor which has to be referenced to barometric pressure. Do some Googling and you will quickly see how it works.
I haven't taken my MAF sensor out to inspect it and am only going by the picture I saw in the DIY. If you're certain that it is not a pressure sensor, then I stand corrected regarding the probability that it was a pressure sensor, though everything else I said still applies as the way it generally operates is the same.
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Old 06-29-14, 12:13 AM   #8
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In an effort to answer my original question, I looked at one of the RX300 service manuals I have (PDF), and found the attached procedures you can follow to diagnose a bad MAF.

Hope this helps any others who encounter a P0171 or are interested in testing their MAF.
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File Type: pdf MAF Testing - Pages from RX300 Repair Manual.pdf (100.2 KB, 48 views)
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Old 06-30-14, 06:50 AM   #9
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I would do a little more investigating before assuming the MAF is the culprit. P0171 is a Bank 1 System too Lean error code. If the MAF sensor was truly the culprit, then you should also be getting a P0174 Bank 2 System too lean code because the MAF is common to both banks. A Bank 1 only system too lean could be caused by many other things such as a bad Bank 1 A/F sensor, exhaust system leak upstream of the A/F sensor, vacuum leak, bad fuel injector on Bank 1.
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Old 06-30-14, 11:50 AM   #10
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Originally Posted by Baetke View Post
I would do a little more investigating before assuming the MAF is the culprit. P0171 is a Bank 1 System too Lean error code. If the MAF sensor was truly the culprit, then you should also be getting a P0174 Bank 2 System too lean code because the MAF is common to both banks. A Bank 1 only system too lean could be caused by many other things such as a bad Bank 1 A/F sensor, exhaust system leak upstream of the A/F sensor, vacuum leak, bad fuel injector on Bank 1.
Not so!! It is very common for a dirty/faulty MAF sensor to throw only the P0171 DTC. Happened to me a couple times and another couple times to other folks I know personally.
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Old 07-13-14, 09:51 AM   #11
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Default Proper resistance for RX300 MAF

Hi all, hoping someone can help by providing some measurements from a working RX300.

Background: I am getting a solid CEL with code P0171 (Bank 1 Too Lean). Having seen many posts identifying a bad MAF (mass airflow sensor) as a possible culprit for P0171, I'm trying to systematically test the MAF to diagnose whether it is bad. I'd prefer not to just replace the MAF unless I am pretty confident it is the culprit. (See my prior ClubLexus post here.)

I have a RX300 repair manual that describes some diagnostic steps for determining whether a MAF is bad. One of the steps it describes is testing the resistance of the MAF's IAT (Intake Air Temperature) sensor. Below are the relevant excerpts from the repair manual. I tested mine and am getting between 5,000-6,000 Ohms at ~ 76 degrees Fahrenheit, when it should really be closer to 2,000-3,000 Ohms.

Can anyone else check the resistance they are getting with a known good MAF and let me know? Should only take a couple minutes and all you need is a (Philips) screwdriver and a multi-meter.

Thanks!

P.S., I have already cleaned my MAF so that should not be a factor.
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Last edited by vishesh; 07-13-14 at 09:54 AM.. Reason: A few typos.
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Old 07-13-14, 10:55 AM   #12
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You have it by the authority of the Service manual. I would not poll, but that is just me.

Is there some other underlying reason that we need to address.

Salim

Last edited by salimshah; 07-13-14 at 10:59 AM..
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Old 07-13-14, 11:21 AM   #13
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Quote:
Originally Posted by vishesh View Post
P.S., I have already cleaned my MAF so that should not be a factor.
Are you sure? Many have mistaken the IAT sensor bulb for the MAF element.
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Old 07-15-14, 07:54 AM   #14
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Originally Posted by salimshah View Post
You have it by the authority of the Service manual. I would not poll, but that is just me.

Is there some other underlying reason that we need to address.

Salim
Salim, good point. But I also purchased a MAF pulled from a working RX300 and got the same measurements. Could be that the pulled MAF I bought was also bad, but really wanted to get a confirmed working datapoint before reaching any conclusions.
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Old 07-15-14, 07:55 AM   #15
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Quote:
Originally Posted by artbuc View Post
Are you sure? Many have mistaken the IAT sensor bulb for the MAF element.
Whoops, now that you mention it, I probably am thinking of the IAT sensor bulb (the amber colored bulb that is clearly visible in the MAF). But I also cleaned the two little wire elements that are hidden deep within the recesses of the MAF. So hopefully that's all there was.
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Old 07-15-14, 07:55 AM
 
 
 
 
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