MAF sensor FAQ, troubleshooting, and replacement for VW and Audi TDI (diesel)

Sep 9, 2016
MAF sensor FAQ, troubleshooting, and replacement for VW and Audi TDI (diesel)
  • TDI engine TDI MAF FAQ for TDI diesel engines 1996+

    This article shows two types of MAF used on VW and Audi TDI, repair/replacement, and MAF info.

    What's a MAF and what does it do?

    All modern VW, Audi, and TDI use a MAF to help measure air flow. A MAF sensor is one way of directly measuring how much air is passing the MAF sensor and into the engine. Some cars, especially older cars may calculate air flow instead of measuring it by using other air flow sensors and taking into account engine rpm, throttle position, and air temperature. Knowing how much air the engine is getting is important because the car's computer uses this value to know how much fuel to give the engine. If it's giving too much or too little fuel the engine won't run right. This is one reason why it's important to have an airtight intake tract after the MAF sensor. The amount of metered air won't be correct if it's leaking out. Air leaks are called vacuum or boost leaks, see 1000q: boost leak test for more reasons why this is bad and how to check for it in your turbo car.

    A faulty MAF signal will cause the car's computer to give too much or too little fuel. However, a TDI can run without a MAF signal at all, as can many cars. The car computer knows that there is either a nonsense signal or no signal at all, and uses a default value to keep the engine running. The computer takes the engine load and speed, modifies the value for some fuel and engine temperature readings and MAF value, etc., to determine fuel injection timing and metering. A gasoline car will run poorly with no MAF because they have to be within a narrower air/fuel ratio to run smoothly compared to a diesel engine. A diesel will run without the MAF signal but performance will be poor and the engine should feel sluggish.

    The MAF sensor on mk3 was a hot wire type sensor. Mk4 and newer use a thin film or hot film MAF.

    Never use silicone detailing sprays or silicone products that have not cured near the intake on a running engine. These could deposit on the MAF and contaminate the sensor. For the same reason, the engine air filter has a gasket on it - make sure the gasket is seated during filter replacement or else dirt can damage the MAF.

    Mk3 TDI
    The mk3 TDI uses a Pierberg MAF. This is a hot wire type MAF. Unlike the mk4 cars, it's a simpler device and it's unusual for the MAF to fail. VW part number is # 074-906-461

    mk3 MAF
    Pin Description Wire color Connected to
    1 5V reference input voltage Red/green pin 50 of the ECU
    2 Signal ground Brown/blue pin 25 of the ECU
    3 12V supply input Yellow/black relay 109
    4 Not connected . .
    5 ground brown ground
    6 Output voltage Purple/brown pin 52 of the ECU

    The hot wire MAF is composed of a thermistor (temperature sensing) wire and an ammeter (measures electrical amps). Since the MAF wire is a "positive k" thermistor, the hotter it gets, the higher its resistance. The colder it gets, the lower its resistance, and the more current that passes through the wire. However, the more current flowing through the wire, the hotter it gets (similar to an electric heater). This sets an upper stable limit of current, since as the resistance of the wire increases, the less current that flows through it. This "stable" point changes depending on how much air is flowing past the wire, providing cooling. The more air flowing past the wire, the more current the wire can allow through before it heats up enough to offset the current. Therefore, at any given time, the amount of air flowing through the MAF is proportional to the amount of current flowing through the wire in the sensor.

    The output voltage increases directly with the measured airflow.

    To clean it, use a non-residue electrical cleaner. These are slightly different than brake or carb cleaner. It's also possible that there is a wiring harness short or corroded plug.

    Mk4 and newer
    Mk4 and newer cars use a Bosch hot film MAF. This is a membrane type MAF and can take much faster and more accurate measurements than a hot wire. It is known for a slow loss of function and causing low power in the TDI and was an extended warranty item on pre 2002 mk4 TDIs. For some reason, failures on mk4 seem to be more common than mk5 or newer MAF.

    The most recent OEM part numbers are # 038 906 461 C for the ALH engine, #038-906-461-B for the BEW and mk5 BRM pumpe duse engines (same part), #074 906 461 b for the 04-05 Passat BHW engine, but check with your VW part dealer to see if this is current. VW has gone through a number of MAF part numbers, the latest part number for the ALH engine ends in "c". The housing is very different but it's not known if it's more robust than earlier designs because the element is what fails.

    MAFs from VW gasser models will bolt on, but I would not use them because they require modification to send the right signal.

    Here is the location of the MAF on a mk4 outlined in yellow. The "chip" element is replaceable separately from the "chip+plastic housing".

    The mk5/mk6 MAF sensor location is shown below. It's also downstream of the air intake box.

    mk4 MAF
    Pin Description Wire color Connected to
    1 Temperature sensor output,
    not used in ALH engine
    . .
    2 12V supply input Yellow/black relay 109
    3 Ground brown pin 4 of the ECU
    4 5V reference input voltage Red/green pin 50 of the ECU
    5 Output voltage Brown/blue pin 52 of the ECU

    A hot film MAF is comprised of a diaphragm with a heating zone and two symmetrically arranged temperature sensors on either side of the heating zone. A control circuit maintains the heating zone at a constant temperature above ambient such that the downstream sensor reads a lower (but constant) temperature, regardless of ambient air temperature. The more air that blows past the upstream sensor, the cooler it is when compared to the downstream sensor. Thus, the change in temperature (ΔT) between the upstream and downstream temperature sensor is proportional to the amount (and direction) of air mass flowing over the membrane. A thin film MAF can also detect a reverse flow of air.

    The graph below of a thin film MAF shows how the temperature is steady before and after the heating zone with no air flow but changes with air flow.


    How to test or replace the MAF sensor

    The mk4 Jetta New Beetle / Golf MAF is outlined in yellow in the picture below. If you buy a new MAF element only it's just the part outlined in yellow, if you buy the housing it's also the hard plastic cylinder.

    The MAF is a common failure part on mk4 VW TDI Jetta, New Beetle, and Golf. It might produce a sudden reduction in car power but the normal failure mode is a gradual loss of power over weeks. If power is restored by shutting the car off and turning it back on, you are in limp mode and the MAF is probably not the main contributor to the power loss. The MAF could be slowly failing and contributing to power loss but if you suddenly go into limp mode, you have other problems. See 1000q: limp mode for more details on diagnosing limp mode. If you have constant low power it could be the MAF, see 1000q: constant low power or low revs for other causes.

    The solution is to replace the faulty MAF. It is covered by an extended warranty for 7 years or 70,000 miles for pre-2002 cars. If you over oil an oiled-type aftermarket air filter, excess oil and dirt will get onto the hot film and damage the MAF. If you re-oil any aftermarket oiled air filter, just put a very thin bead at the top and let it migrate throughout the filter because any more will probably damage the MAF. The mk4 TDIs seem to be very sensitive to contamination and oil due to the MAF design.

    For this reason alone, oiled aftermarket air filters are not recommended. In fact, the stock TDI air filter is the same exact filter used on VW's 1.8T, vr6, and even the 240hp R32. Dyno tests show that there are little-no power gains from non-stock air filters. Only the 90% percentile of the most highly modified TDI would probably see a noticeable benefit. Why do they use a filter with so much excess capacity? Because it's cheaper to make 1 airbox and 1 air filter for all their cars instead of 10 different parts. (They also sell a cold weather air filter so technically it's 2 filters). Side note: the R32 Golf does use a larger air box and cold air intake snorkel but it uses the same exact filter.

    Lastly, a lack of power could also be caused other things so make sure that the engine air intake filter, snowscreen, and intake manifold are relatively clean and operating well. See the FAQ for detail DIY.

    VW has gone through a number of MAF part numbers, the latest part number for ALH TDI engines ends in "c". The housing is very different but it's not known if it's more robust than earlier designs because the element is the part that fails. The newer mk5 BRM engine MAFs look similar but have a different part number.

    Parts (click links to compare current prices)

    Ross tech VCDS cable (For all cars and for 1990-2004 cars) optional but strongly recommended for testing, used to be called vag-com)
    MAF element only removal tool from kermatdi (if you decide to remove the MAF element), a dremel can also be used to cut slots in the screw
    1 MAF sensor: check with your parts supplier to see if there is a new part number
    ALH engine mk4 Jetta, Golf, New Beetle 1998-2003: VW# 038 906 461 c (newest part number)
    Old part numbers for the ALH engine: 06A 906 461, 06A 906 461X (remanufactured - note the "x" suffix)
    BEW and BRM engine mk4-mk5 Jetta, Golf, New Beetle 2004-2006: VW# 038 906 461 b, Bosch#0 281 002 531
    CJAA/CBEA engines for mk5 and newer TDI: VW# 03g 906 461 c

    How to test the MAF mass air flow sensor (for ALH TDI engine but basic principles apply to later engines)

    Note - if your car has an aftermarket chip tune, the MAF requested by the car's computer may be off because the chip tuner changed it.

    If you have really low power the first test is to unplug the MAF electrical plug. Do not remove the MAF from its housing, just unplug the plug connector. The ECU will then replace the faulty signal with a default value. If the car has more power or is unchanged, the MAF is definitely bad. If power is significantly worse, the MAF is good or marginal.

    You can also try driving around with the MAF plug unplugged to see if it makes a difference. Most gasoline cars will run horribly with the MAF unplugged but a diesel car will run OK but slow for a test drive. It may set a check engine light which should go away after you reconnect the MAF and start the car a few times or can be cleared right away with a VCDS.

    Another test is to warm up the car to normal operating temperature and put the transmission in neutral. Rev the engine up to redline in neutral. If the MAF is good, it should go all the way up to the rev limiter at 5100 rpm, if the MAF is bad, it will definitely not rev that high. However, a marginal MAF can also rev that high so to be sure, do more testing with a VCDS cable. The point of doing these tests first is because they will reveal a really bad MAF.

    The most reliable and accurate method of checking the MAF is with a VCDS cable. This will show a MAF working at only 85%. The above tests will only show a major MAF problem. Through VCDS, check MAF actual vs. specified (by the car's computer) at idle, high rpm, and high load. A low MAF actual could also be from servere intake clogging or possibly a malfunctioning EGR recycling too much EGR gas from the exhaust. Start the engine, plug in the cable, and click on "engine", then click on "meas. blocks" in the screen below. Mk5 and mk6 cars are slightly different but the same test applies.

    You should see the screen below. Hit "up" until you see "group 3". Below is an example of a bad MAF (engine is at full throttle) on a mk4 ALH TDI engine - the specified value and actual values are way off. Bad MAFs don't go over 400-500 because the signal degrades. Also check for faulty wiring or a bad plug.

    You can also click "log" to view a graph. Below is a good MAF - the specified value and actual value are close and both can reach 800+. Above 3000 RPM, it should reach about 850. Some spikes at the beginning of each test run are normal due to turbo boost spikes.

    Now do some tests while driving - have a helper operate the computer while you concentrate on driving, don't get distracted while driving or else it could result in a crash! It would be best to find a hill so that you can load the engine and not exceed the speed limit, of course!

    In 2nd or 3rd gear, start at a low rpm and give heavy acceleration to a high rpm near redline. Note the MAF actual vs. MAF requested. You can also do this test in 4th and 5th gear. What you want to see is MAF actual and MAF requested match or be almost an exact match. This means that the sensor is delivering what the car's computer expects. If they are off or MAF actual plateaus then the MAF sensor is bad - replace it. A good ALH MAF sensor should be able to read up to around 850-900 mg/h. I'm not sure the BEW sensor range but it should be about the same. The BRM MAF sensor should go a little higher because the engine has more power. The CBEA and CJAA 2.0L TDI engine MAF sensor should go even higher due to more power = more air needed.

    Below is an example of the MAF reading on a BRM engine.

    Again, it's possible for an EGR malfunction or clogged intake to displace or restrict air. Temporarily blocking off the EGR with a block off gasket will set an error code but it will let you know 100% if the gasses are causing the low reading. Test the EGR valve function before doing this test since if the valve works it's probably not the problem.

    While cleaning a bad MAF with MAF cleaner spray from the local auto parts store won't hurt anything, cleaning a hot film MAF normally doesn't fix it. Just make sure to use a no residue cleaner. It sometimes works with hot wire sensors but the MAF used in your TDI uses a more advanced and more sensitive hot film. It's worth a try.

    Replacing the MAF
    First, check the prices on MAFs. If available, a rebuilt MAF is just as good as a new MAF because they completely replace the MAF element. A rebuilt MAF will only reuse the old plastic housing. If you purchased the element with a housing then just loosen the spring clamps and phillips screws, remove, and replace. When replacing the housing with the element already in it, you do not need to purchase the security bit MAF removal tool.

    If you purchased just the element, you need a bosch MAF removal bit (linked above) to remove the security screws. You can also use a dremel to cut slots into the screw heads and use a screwdriver to remove them. Harbor Freight also sells a set of security/special bits that should include a special torx bit to fit in the special screw.

    Installation is the reverse of removal. You can replace the security screws with any regular fastener like a phillips screw.