Mass Air Flow Sensor Testing,P0171-P0174 System Lean

Mass Air Flow Sensor Testing,P0171-P0174 System Lean

Vehicle: 2001 Land Rover Discover II  Codes: P0171, P0174, TCM Incorrect Gear Ratio
Symptoms: NONE, Slight pre-detonation (pinging) when hot
LandRover 4X4

STRAIGHT TO THE POINT:

The vehicle has no drivability concerns and was hard to diagnosis. The TCM (transmission control module) is setting incorrect gear ration codes which means the transmission is slipping (usually). Then of course P0171 and P0174, lean both banks. Fuel pressure was within specifications (45-50 psi) and the MAF was reading correctly, I thought! I can hook up the scope and perform a snap throttle test. The signal voltage reads anywhere from 3.7-4.0 volts which is ok from my experience. This is why I’m writing this article. The best way to diagnosis these slight differences in MAF signal is with a scanner that has a good graph display. Take a look at the MAF signal graph display below. The MAF signal that’s causing this code to set does not have an obvious problem, the signal looks ok at a glance. The part about the incorrect gear ration codes setting is only related to Land Rover’s. It has something to do with the way the on-board computers operate. A weak MAF signal should not set a incorrect gear ration code, but it’s a very good thing to take note of in case you ever run into something like this again.

FUEL TRIM:


Another good way to monitor this weak MAF signal is through the short term fuel trim. The signal are pretty much in correlation with each other. Fuel trim is a very important data value to monitor and understand when diagnosing automotive drivability problems.  For example take a look at the before and after pictures of the fuel trim graph display above. If you work on these problem enough it’s a good idea to snapshot these type of value displays for quick reference later on. When you do eventually run into a similar problem it will save a lot of time, but the key word is quick reference. If it takes you too long to pull up these snapshots then it defeats its purpose.

Short term fuel trim is also very good for diagnosing vacuum leaks. Monitor your fuel trim at idle, if you engine is running lean your fuel trim is high. Raise engine to 2500 RPM’s, hold and re-check. If the fuel trim comes down chances are you have a vacuum leak. The reason for this is your vacuum is lower at off idle and therefore less un-metered air is getting in to the cylinders. If the lean condition was caused by low fuel pressure, MAF/MAP sensor, A/F sensor, ect……. The fuel trim would not decrease when at 2500 RPM’s. Most likely the fuel trim would increase depending on the actual problem present.

DIAGNOSTICS FlOWCHART P0171-P0174:

A code P0174 may mean that one or more of the following has happened:

•The MAF (Mass Air Flow) Sensor is dirty or faulty. Note: The use of “oiled” air filters can cause the MAF to become dirty if the filter is over-oiled. There is also an issue with some vehicles where the MAF sensors leak the silicone potting material used to protect the circuitry.

•There could be a vacuum leak downstream of the MAF sensor.

Possible Solutions

Possible solutions include:

•In the vast majority of cases, simply cleaning the MAF sensor does the trick. Consult your service manual for its location if you need help. I find it’s best to take it off and spray it with electronics cleaner or brake cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it’s dry before reinstalling

•Inspect all vacuum and PCV hoses, replace if necessary

•Check for a dirty fuel filter and proper fuel pressure

System Too Lean (Bank 1)

What does that mean? Basically this means that an oxygen sensor in bank 1 has detected a lean condition (too much oxygen in the exhaust). On V6/V8/V10 engines, Bank 1 is the side of the engine that has cylinder #1. The P0171 is one of the more common trouble codes. This code is triggered by the first downstream (front) O2 sensor. The sensor provides a reading of the air:fuel ratio leaving the engine’s cylinders, and the vehicles powertrain/engine control module (PCM/ECM) uses that reading and adjusts to keep the engine running at that optimum ratio of 14.7:1. If something is not right and the PCM cannot maintain the 14.7:1 ratio, but rather there is too much air, it triggers this code. You’ll want to also read our article on short and long term fuel trims to help understand the operation of the engine. Note: This DTC is very similar to P0174, and in fact your vehicle may show both codes at the same time.

Symptoms:

You will more than likely not notice any drivability problems, although there may be symptoms such as:

• a lack of power

•detonation (spark knock)

•rough idle

•hesitation/surge on acceleration.

Causes:

A code P0171 may mean that one or more of the following has happened:

•The MAF (Mass Air Flow) Sensor is dirty or faulty

Note: The use of “oiled” air filters may cause the MAF to become dirty if the filter is over-oiled. There is also an issue with some vehicles where the MAF sensors leak the silicone potting material used to protect the circuitry.

•There could be a vacuum leak downstream of the MAF sensor

•Possible cracked vacuum or PCV line/connection

•Faulty or stuck open PCV valve

•Failed or faulty oxygen sensor (bank 1, sensor 1)

•Sticking/plugged or failed fuel injector

•Low fuel pressure (possible plugged/dirty fuel filter!)

•Exhaust leak between engine and first oxygen sensor

Possible Solutions:

A lot of times, cleaning the MAF sensor and finding/fixing vacuum leaks fix the problem. If you’re on a tight budget, start there, but that may not be the fix for certain. So, possible solutions include:

•Clean the MAF sensor. Consult your service manual for its location if you need help. I find it’s best to take it off and spray it with electronics cleaner or brake cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it’s dry before reinstalling

•Inspect all vacuum and PCV hoses, replace/repair as required

•Inspect all hoses and connections in the air intake system

•Inspect and/or test the intake manifold gaskets for leakage

•Check for a dirty fuel filter and proper fuel pressure

•Ideally you’ll want to monitor short and long term fuel trims using an advanced scan tool

•If you have access, you may want to run a smoke test

Transmission Codes are set from faulty MAF Sensor for the Land Rover – TCC Lockup and incorrect gear ration codes so if you have transmission codes ck the MAF Sensor. I have even seen it where there is no drivability concerns at all and you scope the sensor and it check fine this is where you have to dig deep into the diagnosis and I have shown  you how.

MASS AIR FLOW SENSOR OPERATION:

Check engine light faults relating to the Mass Air Flow Sensor are becoming less common, but do occur. The tricky thing with some of these types of faults is that you can have a mass air flow sensor concern without triggering the check engine light.

Before we get into diagnosis let’s start with a brief overview of the sensor itself. The main purpose of the Mass Air Flow (MAF) Sensor is to measure the volume and density of air entering the engine at any given time. The computer uses this information in conjunction with input from other sensors, to calculate the correct amount of fuel to deliver to the engine. Input from this sensor is also used indirectly to help calculate desired ignition timing and transmission operating strategies. MAF sensors are mainly designed as a “hot wire” sensor or a “hot film” sensor. Both sensors function in a similar fashion. Hot wire sensors pass current across a platinum wire and hot film sensors pass current across a foil grid. The current level is regulated to maintain the hot wire, or film, at a predetermined temperature. This temp is either a direct value, or a value that is a set number of degrees above ambient (outside) air temperature.

So how does this tell us how much air is getting into the engine? Well, as air passes across the mass air flow sensor, it cools the hot wire, increasing the amount of current needed to keep that wire at the specified temperature. The amount that the wire is cooled is directly proportional to the temperature, density and humidity of the air passing through the sensor and as such, the current increase needed to heat the wire allows the computer to easily calculate the volume of air entering the engine.

Diagnosis:

Mass air flow sensors typically either send a voltage, or frequency signal to the powertrain control module (PCM). Hot wire sensors typically have an operating range of 0 – 5 volts, with idle voltage being around .5 – .8 volts and full throttle application is normally between 4 and 5 volts. Hot film sensors typically produce a frequency output of 30 – 50 Hz with 30 Hz being idle and 150 Hz at full throttle. There are some other subtle differences between the sensors but these do not affect function or purpose.

So what types of symptoms can we get from MAF sensors, and how should we test for these faults? Well as we stated earlier, MAF sensors can produce drivability symptoms without generating a check engine light code, so some specific checks are in order. For ease of diagnosis a scan tool should be used to monitor sensor readings. In some instances it’s okay to take sensor value readings by back probing the appropriate terminals at the MAF sensor. If specific MAF check engine light codes are present, then proceed with the appropriated tests. If no codes are present, or if you have lean codes you suspect are caused by a faulty mass air flow sensor, perform the following. Obtain sensor specifications from a reliable source; you can send us an e-mail from the Help Link and we can assist with most information. Hook up a scan tool with the ability to monitor sensor valves (PIDS) and pull up the mass air flow sensor. Record your MAF sensor reading at idle, and again at various RPM ranges. Compare the values against specifications. Next, start from idle and increase throttle opening while watching MAF reading. Increase should be steadily proportional to RPM change. Perform the same checks while lightly tapping on the sensor, or heating the sensor with a blow dryer. Any fluctuation, or out of specification reading indicates a mass air flow sensor, or related wiring concern. Repair and retest. I would also recommend monitoring MAF values while driving the vehicle and checking readings while the concern is present. Have an assistant drive while checking these readings. If the mass air flow reading is within specification while a concern comes and goes then it is not likely the problem. Be sure to check all air intake connections and gaskets, as well as the air filter before faulting the sensor, as these types of concerns can affect readings.

sparkaccess.jpg

Land Rover/Intake Hose Removed

MAFremoved.jpg

Mass Air Flow Sensor Removed

Conclusion:

it is not always necessary to replace a mass air flow sensor that is reading out of specification, though most dealers will tell you differently! It is possible that the sensor is just contaminated from age or use of oil saturated, aftermarket air filters. You could try exposing the sensor hot wire (once the sensor is removed from the vehicle) and cleaning it with electronic parts cleaner and low pressure air. Use appropriate cautions. Once the sensor is clean, reassemble and install and check operation, you may be pleasantly surprised! I hope this information has been helpful. Thanks for visiting , and have a great day!

This post was written by: Martin Hand

Donations

If you find this information helpful please consider a donation. These articles, questions and comments are very time consuming so even a small donation gives me motivation to keep educating automotive owners. Donations will allow us to continue open questioning/comments, automotive education and repair tutorials in the future as the business grows. All proceeds go to the expansion and maintenance mdhmotors.com. Thank You



Martin Hand

About Martin Hand

ASE Certified L1 Advanced Mastertech. Martin Hand has over 15 years experience in Asian and European Import Auto Repair. Specializing in electrical diagnosis, engine performance, AT/MT transmission repair/rebuild. Martin is also pursuing a degree in Computers Science & Information Systems starting at Portland Community College while he plans to transfer to OIT. Certified in Java application level programming, experienced with other languages such as PHP, Ruby, JavaScript and Swift. Martin has future plans of automotive diagnostic software development.

66 Comments

  • Larry says:

    My 2006 Land Rover lr3check engine light stays on . Changed the hydraulic hose across the fuel rail. Still have the light on. Cleaned the MAF AND CHECKED THE HOSES connected to all the engine . Pcv and the like. I even sprayed all the parts while the engine
    Was running, still didn’t fix the problem. Will take back to the shop this week, to purchase
    The fuel rail Assy.

  • John says:

    I have an 06 Mustang GT with a Roush supercharger setup I installed in 09.
    The problem its having now is specifically that it stumbles only right after startup, only when hot and only after hot and been sitting for around 20 mins. The stumble will occur right after the engine revs up at start and usually smooth out after 8 seconds. bad stumble incidents will cause code P0171 and set of CEL.
    At first, from what I supposed the issues to be, I changed the MAF, fuel pump and fuel filter.
    Then with no change whatsoever to the symptoms, I took the car to a shop for diagnosis. He checked it for 2 hours. His conclusion was to clean the fuel injectors which I did. I removed them and had them professionally cleaned. There is still no change and Im right back at square one.
    I called the tech who did the diagnosis and told him the injector cleaning didnt work. all he could think was to have the PCU reflashed because he said all the components seemed to be functioning. I called Roush who flashed my PCU in 09 when I installed the supercharger kit and inquired about them reflashing the PCM because the tech thought there might be a glitch in the program. they said they would flash it for $100. I asked if this is something they are ever asked to do under the circumstances and he said almost never. So i am trying to figure this out knowing a little about how this system works, but not yet enough to put all the pieces together to figure it out.
    If I may share, here is the diagnosis from the tech:
    “Work Performed: As car warms up, the fuel trims climb. They started at 9-12% and climbed to 25% +. Checked the fuel pressure sensor and was reading correctly. Computer readout compensates for vacuum and pressure. The computers goal is 39 psi difference from atmospheric pressure. At idle the computer reads 39 psi and the actual pressure is about 30 psi. with thw vacuum hose disconnected from the sensor, the pressure rises to about 40 psi. 02 sensors switching and working good. I detected no alcohol in fuel. Recommended cleaning injectors and recheck system”

    So I still have same original problem. no changes.
    I have print outs of the freeze frame of the error code and printouts from the tech at idle warm up and at 3000rpm. It does show the long term fuel trims in banks 1 and 2 rising and other info Im not quite savvy enough to figure out.
    . the error never occured any time but at hot startup and after sitting for about 20 mins.
    any ideas?
    Thanks, John

    • Martin Hand Martin Hand says:

      Probably have leaky injectors or fuel pump check valve bad bleeding residual fuel pressure which also gets air in the line. Sounds to me like it’s getting loaded up or leaned out after the sit. Fuel boiling on hot days will cause this also.

  • karen says:

    I have a 2004 Ford free star having problems with it cutting off. Replaced the maf it was running better than the old one. It is cutting off again. I replaced the fuel filter as well. It was reading p1071 which said replace maf. I did it’s still doing the same thing please help.

    • Martin Hand Martin Hand says:

      P0171 doesn’t mean replace the mass air flow sensor, it’s referring to a lean condition. Use a scanner to monitor the fuel pressure (Ford uses a fuel pressure sensor) or check it with a gauge. I would bet this is your problem from what your explaining. I have replaced lot’s of Ford fuel pumps, it will be a complete module assembly in the tank.

      • karen says:

        I got the fuel pump at first then the mechanic say it wasn’t the fuel pump it was the fuel filter I replaced that. I was told to take it to auto zone to put it on the ferr testing machine and it showed those codes he typed it in and it printed out replace the maf. So I did

  • […] It starts kind of techy, but it gets easier after the graphs….hope it helps. Mass Air Flow Sensor Testing,P0171-P0174 System Lean | MDH MOTORS […]

  • Mona says:

    My 2006 Range Rover Sport check engine light came on and scanned p0171. Took right in. No other symptoms. Mechanic called said he started it, let it run for 10 minutes. Walked away from it, turned around it was bellowing smoke and a piston blew and is sitting in the passenger side of the motor. Did they cause this?

    • Martin Hand Martin Hand says:

      I doubt it, from the explanation they were just letting it idle. Does seem kinda odd though, would think you would have had previous symptoms but sometimes mechanical failure happens without warning.

  • janeesh says:

    Honda Civic 2013. Some time start and sometime only crank. Scanner telling maf sensor reading – volt

  • […] Mass Air Flow Sensor Testing,P0171-P0174 System Lean – Mass Air Flow Sensor Testing,P0171-P0174 System Lean Vehicle: 2001 Land Rover Discover II Codes: P0171, P0174, … MASS AIR FLOW SENSOR OPERATION: […]

  • Linda says:

    My engine light came on. I had the codes checked and they came up p0101 and p0171. I tighten the gas cap and the light went out. However I was told that the code did not mention gas cap. When I called the auto shop they said that they have to do a dianosis even though the light is not on now. They also said that the car has to be leftover night because it has to be cold when they check it. Am I wasting money since the light is not on. Should I drive it to see if the light comes back on or should I just have it diagnosed. Money is very tight and to pay $100 for a dianosis test is something I cannot afford. However I don’t want to cause futher damage. I was told that Auto Zones codes are crap and that he will not go by what they say. I need to get a E test and I am worried it won’t pass. I was told that if I drive it for a few days that the code will correct its self and I will pass the E test. Pep boys said that it will not show a code if the light is out. Does that also affect the dianosis test. If it fails a E test do I still need to pay for a diagnosis or can they go by what the fail test said without a dianosis? I don’t want to be charged for something that may not be needed in order to be fixed. I hope this makes sense.

    • Martin Hand Martin Hand says:

      The check engine lite will go off but the codes will still be stored in memory. The problem could be intermittent so the lite may come on and off but codes will still be stored in memory unless cleared. Autozone will scan generic OBDII codes, they work okay but diagnostics testing still needs to be performed. I would recommend to have a technician do a proper diagnosis on the vehicle.

  • akil rav says:

    Hey i have a 2003 range rover hse

    i have five codes currently:
    p0130 = oxygen sensor circuit bank 1
    p0174= bank 2 system too lean
    p0160 =oxygen sensor circuit no activity detected bank 2 sensor 2=
    p0171 = bank 1 system too lean
    p1174 = no dtc no defintion found

    before these codes came up i only had the p0171 and p1174 , so i took out the o2 ssensors and cleans then and reinstalled them , then i got the 3 other o2 sensor codes, so i bought some o2 sensors today and install them and im still getting the 5 codes above , i really need help. what is the problem with my 03 range rover ?

    • Martin Hand Martin Hand says:

      The fuel trim codes indicate a vacuum leak or some other condition causing it to run lean. The O2 sensors are most likely okay. I would suspect a vacuum leak or weak MAF signal.

  • James says:

    I have a 2001 Silverado with lean codes and a abs code. Changed maf, fuel filter check fuel pressure and smoke test for vacuum leaks. Also changed pcv and air filter. As well as tune up. Codes still come back. Any in put?

    • Martin Hand Martin Hand says:

      Monitor your fuel trim values to see why its setting lean. You should be able to tell if it’s a vacuum leak or fuel supply problem with short term fuel trim values. The engine is low vacuum under load part throttle so if the short term fuel trim doesn’t change under this condition I would suspect fuel supply problem and vise versa.

  • awani says:

    I have a range rover 2011 that shows P0171 P0174, it ramps rpm to deliver high speed above 70rpm and sometimes show POOR performance when fuel tank level is below 40percent.

    Do you think it’s an issue with fuel filter

    • Martin Hand Martin Hand says:

      I wouldn’t suspect the fuel filter unless I actually seen the weak fuel pressure with a gauge. Something is causing it to be lean, vacuum leak or MAF sensor dirty/failing are more possible. Need to use a scanner and monitor the short term and long term fuel trim to diagnosis a problem like that.

  • I’d like to thank you for the efforts you’ve put in penning this
    blog. I am hoping to view the same high-grade blog posts by you in the future as well.
    In truth, your creative writing abilities has motivated me to get my own website now 😉

  • geoff says:

    Ps I put the range rover on a snap solus on diag and it gave two errors.
    P0102 mass or volume air flow circuit low input
    P007F charge air cooler temperature sensor bank 1 and 2 correlation.
    Previously, before I cleared codes there were various injector codes but I wondered if these were created due to the lumpy running.

    • Martin Hand Martin Hand says:

      I doubt it, the injector codes are only set for open circuit of high resistance. Either injector coil winding is going bad or the codes were set during testing, maybe had them unplugged somewhere along the line?

  • geoff says:

    Great information very helpful.
    I have a 2007 3.6 V8 will lumpy power and idle intermittently. When the lumpyness occurs i get white smoke out of the offside exhaust. This lumpy power and idle goes away and it runs perfectly when I disconnect the two MAF’S. I have cleaned the MAF’S with cleaner but no change. I am considering replacing one or both MAF’S but fell concerned that a vac leak could be the cause but I’m not sure how to find it. On my 3.6 V8TDI there is hooked stiff black plastic pipe with hook end disconnected and doest look like anything was connected to the end and I can’t see anything that is nearby to connect to it. This pipe can be seen as when the air box top is removed and the two air intake pipes are moved aside this hooked pipe sits in between the two rubber air intake pipes. I have never come across a pipe like this so feel suspicious about it. I need to know if this hooked pipe is normal or do I need to look deeper. When the range rover is running lumpy people tend to say it’s an injector sticking but this sounds unlikely as the lumpyness goes away instantly as soon as I disconnect the MAFF’s and the car runs smoothly again until I reconnect them and the smoke and lumpyness comes back. Any help would be much appriciated.

    • Martin Hand Martin Hand says:

      You would need to look at the fuel trim values to determine if its a lean or rich condition. Do some research on how long term and short term fuel trim work, they are the same for all manufacturers. Some manufacturers use different terminology.

  • geoff says:

    Correction of last paragraph:
    When the range rover is running lumpy people tend to say it’s an injector sticking but this sounds unlikely as the lumpyness goes away instantly as soon as I disconnect the MAFF’s and the car runs smoothly again until I reconnect them and the lumpyness and smoke comes back. Any help would be much appriciated.

  • geoff says:

    Great information very helpful.
    I have a 2007 3.6 V8 will lumpy power and idle intermittently. When the lumpyness occurs i get white smoke out of the offside exhaust. This lumpy power and idle goes away and it runs perfectly when I disconnect the two MAF’S. I have cleaned the MAF’S with cleaner but no change. I am considering replacing one or both MAF’S but fell concerned that a vac leak could be the cause but I’m not sure how to find it. On my 3.6 V8TDI there is hooked stiff black plastic pipe with hook end disconnected and doest look like anything was connected to the end and I can’t see anything that is nearby to connect to it. This pipe can be seen as when the air box top is removed and the two air intake pipes are moved aside this hooked pipe sits in between the two rubber air intake pipes. I have never come across a pipe like this so feel suspicious about it. I need to know if this hooked pipe is normal or do I need to look deeper. When the range rover is running lumpy people tend to say it’s an injector sticking but this sounds unlikely as the lumpyness goes instantly as soon as I disconnect the MAFF’s and the car runs smoothly again until I reconnect them and the smoke goes away. Any help would be much appriciated.

  • Michael Wanzek says:

    Hello, My check engine light came on, so i bought a code reader and said 0171 and 0174, asked around and was told the mass air flow sensor. replaced it and the pvc valve the code come off by its self. okay sweet. very next day started, same codes came up. Check for leaks, N/a. cleaned the intake what i can reach form the front. Gas lines okay. change gas filter last month. So i am asking if i should change out Bank 1 sensor and go from there?

    • Martin Hand Martin Hand says:

      No, the O2 sensors are probably working since it’s setting the lean codes. If it’s an AF sensor then that’s a different story, I would need to know the vehicle type. Monitor the short term fuel trim to see if it’s a continuous lean condition or only lean off idle. If you don’t have a scanner to monitor fuel trim then look for vacuum leaks and check the fuel pressure.

  • Don Decker says:

    Hi- I came across your article about the Mass Airflow Sensor because my 2002 LR Discovery II has been having a stalling problem. It mainly occurs when I come to a stop or accelerate form a stop. The vehicle stalls, I drop into neutral, and it starts right back up. I did a diagnostics and pulled the following codes:
    P0134, P0141, P0101, P0174,P1884 and P1590. Need some advice. Is the stall
    Inn caused by a bad Mass Airflow Sensor?

  • Ray says:

    Thank you for an informative knowledge base for the po171, I have the lexus gs400, I have done all the above testing and vac leak repairs,ie spray at the manifold etc. my long term fuel is 34 on bank 1, and does come down as rpm increases, i have ran fuel injector cleaners etc. my memoscan scan tool cant read maf in lbs per minute, do i need a better scan tool? thanks

    • Martin Hand Martin Hand says:

      I would try cleaning the mass air flow sensor. If the fuel trim is not consistent with a vacuum leak then most likely the problem is with the mass air flow sensor. Still cover your basics like fuel pressure.

  • Manmeet says:

    How MAF sensors helps to calculate ignition timmimg??

    • Martin Hand Martin Hand says:

      It’s done within the logic of the vehicles powertrain controller. The PCM will calculate load and determine ignition timing from there. MAF is only one of the sensors used to determine this and every manufacturer is different.

  • 1 2

Leave a Reply

Your email address will not be published. Required fields are marked *