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


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.


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.


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.


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.


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.


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.


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.


Land Rover/Intake Hose Removed


Mass Air Flow Sensor Removed


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


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 Thank You

MDH Motors logo

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.


  • franko says:

    p0171 .if it is bank 1….only bank 1 how is it the maf.wouldnt it be a problem for both banks lean ?

    • Martin Hand says:

      Depending on the engine, if it’s inline 4 or 6 cylinder there is only one bank.

      • franko says:

        thanks for responding. im talking v8. went to aurozone w the code and was told gas cap maf…but how is it im lean in bank 1 LTFT ABOUT 28 AT IDLE..Bank 2 is around 7.both banks regulated/use maf/gas.both banks would be affected close to equal i would think.

        • Martin Hand says:

          It’s something else then, probably vacuum leak. Watch STFT to see if value changes to normal range at cruise speed this will indicate a vacuum leak. There is low vacuum at part throttle and STFT reacts immediately so that would indicate vacuum leak otherwise make sure upstream O2SB1 is not stuck on higher voltage (rich signal).

  • Thiis is really interesting, Youu are a very killed blogger.
    I’ve joineed your rss feed and lookk forward to seeking more
    of your excellent post. Also, I have shared
    your site in my social networks!

  • Gerry Eisert says:

    I have a 2011 Subaru Forrester…. same diagnosis. Where do I start. I understand Subarus are notorious !

  • saad says:

    2008 ford explorer
    running rich, lose power, black smoke
    fuel trim bank 1( 66%), fuel trim bank 2 (-29%)
    cleaned egr, maf, pcv and injectors.
    please help

    • Martin Hand says:

      I would need more information to help. I have seen stuck open fuel injector cause similar problems on these Ford’s. Also check PCV hose and possibly other vacuum leaks.

  • Troy says:

    I have a 1998 dodge ram 1500 5.2 liter 5 spd.
    I was having map sensor out of range codes. Down the road i started having lean bank 1 code causing bad fuel mileage. I replaced the o2 and fuel mpg returned to normal but not long after started throwing map code again. Replaced map sensor, checked for vacuum leaks, replaced pcv valve etc. I have 5 volts at map sensor on power wire but with key on only have 3.7 volts return. The repair manual says i should have approx 4.5. This voltage is exact same with both old and new sensor. When starting the truck voltage drops to 1.3 as it should but when i accelerate instead of the voltage raising like it should it drops off below 1 volt. This happens with both old and new sensor. I checked for resistance in the plug for the map sensor and it was fine. I unplugged harness going into pcm and checked and no resistance as well. Help! Also a side note the cel is not always lit just sometimes. Could this be a bad pcm or what should i do? Thanks in advance.

    • Troy says:

      Also i have checked fuel pressure as well. No exaust leaks also.

      • Troy says:

        Also i have checked the ground and it shows 12 volts with ground wire backprobed on map sensor and other probe on positive battery terminal.

    • Martin Hand says:

      Map sensor signal as follows: 0-5v signal wire, ground and 5v reference wire. Sounds to me like your checking the signal wire. Low engine vacuum = low voltage, high vacuum “engine idle” = high voltage. Hope this helps

  • sandro says:

    Bmw e60 m54 how much grams per second should i see on obd2 on idle ?

  • 2001 toyota corolla with a P0171 lean bank 1 code.
    Car starts and runs good with good power. I have a graphing code reader.
    Short term fuel trims seem normal at + or – 5 to 8% and averaging close to 0 as they should. Long term fuel trims are +32 coasting downhill with a closed throttle and +38 to + 42 anywhere from a standing still idle to full throttle under load going up a hill.
    I have checked carrfully for vacuum leaks and found none including the intake manifold and all hoses. Tested with propane and with starting fluid both listening to idle as well as watching the front O2 sensor on a scope, no sign of a vacuum leak. Plugged PVC, and valve cover vent connection, brake boost vacuum connection, and fuel canister vacuum connections, to the intake. No change in long term fuel trims. Still 38 to 42, except coasting downhill with throttle closed when long term drops to about +32. No exhaust leaks between engine and O2 sensor. Fuel pressure in spec from idle to full power. Changed pvc valve. No change in long term fuel trim. Short term trim remained normal.
    Changed MAF sensor with two different used sensors (all cleaned carefully with MAF cleaner) and finally 1 new one with only a slight change (1 or 2 %) in long term fuel trim with any of them. Changed front O2 sensor. Short term fuel trim still normal and long term trim the same high positive values give or take a percent. Both O2 sensors show normal response on scope. Air filter is clean. Tested briefly with no air filter and did not change the high long term fuel trim more than a couple %.

    I can erase codes but check engine light and P0171 code will return in a few miles, as long as long term trim is high.

    • Martin Hand says:

      I have seen this before and had to replace the injectors. As long as all your others tests are accurate. I have also fixed them by doing a professional fuel injection cleaning (Motorvac by SnapOn). If it was a vacuum leak you would see changes in short term ft also.

  • Sam says:

    Good day to you Martin

    1) can we clean the sensor and the wire with brake cleaner, or does it have to be electronic parts cleaner !

    1) I have a Toyota yaris 1999 1l cdx model . Engine light came one, checked for codes and it was code 26 . ( running Rich) . I cleaned the air flow sensor sensor with wd40 and did reset the ecu to clear the code . Drove the car for a while,, engine light came on again while driving . Checked for codes and this time got code 25 ( Lean running ). Any advise on this strange behaviour . Kind regards and happy xmas.

    • Martin Hand says:

      Don’t clean it with WD40 use electrical parts cleaner. I have used brake cleaner in the past without problem but recommend to use electrical parts cleaner or even MAF sensor cleaner.

  • Shawn says:

    Hi I’m having problems driving up a hill, my can only goes like 20kmh and jerks back and forward but works fine on flat road, no loss of power on flat roads or speed. Pleases help me.

    • Martin Hand says:

      When the loss of power occurs does the vehicle misfire (would be shaking or sputtering) or just bog down? I would check fuel delivery for bog down symptom and secondary ignition if seems to misfire and loose power. Check for diagnostic trouble codes also.

  • sad says:

    I have a 2003 Ford Expedition 4.6 liter engine I had a check engine light on so I took it and got the code red and it said I needed a mass air flow sensor I replaced it and now when I drive the truck it will stop at all stop signs or red lights can you please help figure out what to do!

    • Martin Hand says:

      If it didn’t stall before replacing the MAF sensor then make sure you don’t have air leak between sensor and engine. Also make sure installed correctly and check the electrical connector, may have broken wires at the connector. Can always try re-installing the old one to see if stalling goes away.

  • RIGGED says:

    Here’s what I’ve got.
    1999 Ford Ranger 3.0
    STFT at idle under 10% but rise with RPM increase.
    LTFM at idle 25% lower to under 10% with RPM increase.

    Smoke test revealed exhaust leak before cat, large leak at vacuum ball reservoir…both repaired.
    Fuel pressure good.
    Only unmetered air leaks-throttle body, IAT, and EGR.

    Will those minor leaks cause this?

  • kevin says:

    i got a 2003 Saturn L300 3.0 v6 111,000 miles on it. its throwing out these codes p0171, p0174, p0300 sometimes and sometimes p0455. i put a new gas cap on it and cleaned the MAF sensor. I fixed a line that was broke by the gas tank. what else could it possible be? I do need to change the spark plugs and timing belt soon i bought the car when it had 74,000 miles on it. do i need to replace the MAF sensor?

    • Martin Hand says:

      The MAF sensor could be causing the P0171 and P0174 codes but I would make sure there is no vacuum leaks first, that is more likely being you cleaned the MAF with no improvement.

  • 1 2

Leave a Reply

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