Mass Air Flow (MAF) Sensor

The Mass Air Flow Sensor is probably the best way to measure the amount of air an engine takes in (engine load). This sensor not only measures the volume of air but also compensates for its density as well. Ford, GM, and many imports are using engine control systems based around this sensor.

Typical Mass Air Flow Sensor


There are two common designs of MAF sensors used in today's vehicles. One produces a variable voltage output (analog) and the other produces a frequency output (digital). In either case their operation is similar. Both outputs can be measured by a scanner or a digital volt/ohm meter (dvom) that can measure frequency.

Both designs work on the "hot wire" principle. Here's how they work. A constant voltage is applied to the heated film or heated wire. This film or wire is positioned in the air stream or in an air flow sampling channel and is heated by the electrical current that the voltage produces. As air flows across it, it cools down. The heated wire or film is a positive temperature coefficient (ptc) resistor. This means that it's resistance drops when it's temperature drops. The drop in resistance allows more current to flow through it in order to maintain the programmed temperature. This current is changed to a frequency or a voltage which is sent to the computer and interpreted as air flow. Adjustments for air temperature and humidity are taken into consideration since they also affect the temperature of the heated wire or film.

Typical MAF Sensor Cross Section


GM (Bosch) Hot Wire MAF Sensor


Humidity always affects the density of air since humid air is denser than dry air. No other compensation is therefore needed for this factor. Air temperature affects density since colder air is more dense than warmer air. Many systems use an air temperature sensor to compensate for this factor since similar amounts of air can enter an engine at different temperatures. Some MAF sensors use an internal "cold" wire to send ambient temperature information to the computer. Some use an intake air temperature sensor in the manifold or the intake piping. This sensor is almost always ntc in design (negative temperature coefficient). That is, it's resistance goes up as air temperature goes down. This "thermistor" works just like a coolant temperature sensor and usually has identical resistance to temperature values. By the way, these values are very different from manufacturer to manufacturer and are available in most repair manuals. They are also programmed into scanner software.

Ford Hot Wire MAF Sensor


Ford Hot Wire MAF Sensor


Now, as we discussed, the MAF sensor sends either a variable voltage or a changing frequency to the computer. The computer is programmed to accept this information when the car is running in any mode. For example, idle rpm will send a low voltage or low frequency and a high revving engine will send a high voltage or high frequency to the computer along a specific wire (the MAF signal wire). If the signal is not present when it should be and within a programmed parameter, say high voltage at high throttle opening, the computer will set a code.

So, there are several things to consider whenever there is a code which points to the MAF sensor as the problem:

1. Derive the code(s) by the manufacturer's recommended method.

2. Look up the code(s) in a service manual.

3. Read the explanation(s) carefully!

4. A code that indicates an out of range signal is often an indication that another sensor, like the throttle position sensor or the rpm input signal is contradicting the MAF signal. The cause might be the other sensor or signal being out of adjustment or faulty.

5. A code that indicates a low MAF signal may be set by various problems. These include the following:

1. A bad MAF sensor (internal fault)

2. Any wire on the MAF sensor circuit including:

A. The 12 volt feed wire which connects the MAF to the battery through the ignition switch or through a relay as in many GM applications

B. The MAF ground wire

C. The output wire

D. The MAF or computer connectors

E. The computer


MAF theory and facts
Article by: Richard MacCutcheon


The following is a compilation of information about MAF sensors and their function from mostly WWW and mail list information. It is biased towards the Ford style two wire MAF used on mostly any Ford since the early 1990s gasoline engines. The MAF sensors used on many of today's cars primarily use the two-wire, hot/cold wire, setup to detect the mass of the air passing through it. These wires are designed to give a feedback voltage to the engine control system (EEC, PCM, ECM, whatever) using an electronic function wired into the sensor. Many misconceptions have arisen in the realm of modifying the MAF to gain power on the vehicle. Here are some facts and information to help you better understand the use and function of the system. MAF electronics, black areas are carbon ink resistors that are grooved to tweak the outputs. The MAF is a sensor. It cannot take an active role in the operation of your vehicle, which is what the PCM does. The PCM uses all of the various inputs and runs the engine according to its program. The MAF transfer function not only tells the computer how much air is coming into the motor, but the function is used to calculate load using other inputs which profiles spark and fuel. As far as sensors go, it is a somewhat complex unit. It basically uses two wires within its body to create a current draw and output based on the different characteristics of the wires. But the way these wires act in the sensor body itself can be somewhat problematic.


The sensor wire at high magnification. Ceramic bobbin with wire wound coated with glass material. First, there is the flow of air entering the sensor. The air stream can have many influencing characteristics that can affect the sensor's ability to give accurate output. Turbulence and velocity variations can give false output based on the original flow profile of the sensor. So let's talk about the design for a second. When the engineers at the factory incorporate the sensor into an intake system, they must not assume it to be a perfect flow of air. The air filter, air box, sensor placement and many other factors affect the true sensing capability of the sensor. This is why when a MAF transfer function, the data stored in the computer about the sensor, is derived for one vehicle it may not be identical to the transfer function for a similar vehicle with the same exact sensor. The load calculations will also vary. Likewise, the downstream inputs that can affect the sensor vary from one design to another.

A running engine creates impulses that flow upstream, intake growl noise, that are waves of air that are now impinging on the sensor from behind. In an effort to eliminate the effect of various intake feedbacks the designers use a backflow-preventor in the form of a bar across the MAF sensor cavity using a backing plate to deflect these impulses away from the sensor elements. When the engineers develop the transfer function for a system they measure the flow of the sensor in a real-life situation.


Designers actually use a laser Doppler system to precisely measure the flow characteristics. Designing a single basic design that can be used across many applications is the goal so costs can be minimized. Suffice it to say that a MAF from identical cars will not be identical. The variations in the electronics that drive the sensor have some influence on the way the car runs. As a model year progresses other variables, like part lot numbers and such can cause one car to run much better than the next. If all of the variation of the sensors and devices like injectors are taken into account, you could have a marginal car to begin with. This is why some people see great gains with slight mods and others see nothing or it gets worse.

Modifying the MAF has risks as well as potential benefits. Changing the flow characteristics can cause great problems. First, let's look at the post removal issue. Removing the post does increase the flow capacity of the meter, but now has the potential for noise from the intake causing other problems. This is more important at idle and low throttle settings. Why? The MAF transfer function is not linear.

The function is flatter at low flows and increases at an increasing rate as flow increases. If there is airflow noise at the flatter portion of the curve, the noise voltage is a much greater percentage of the total voltage being sent to the computer and the computer may balk. Thus you end up with rough or poor idle. At high flows the noise can be greater but the curve is exponential and the noise is less influential.

Also, the velocity of the air in the sensor with no post has dropped at the same load. This affects the load calculation that causes the injector pulses to be off the intended design. They may be better they may be worse. Cars with closed loop operation capabilities can detect the lean or rich condition caused by the change and make adjustments on the fly. Car may start rough but smooth out as you drive. As you drive in the Wide open throttle condition, open loop, the car is now only relying on the data stored in the computer and modifications, which are not "good" or beneficial, will now affect the performance of the motor. The one perceived increase in performance has to do with the fact that the original programming of the PCM is on the rich side, extra fuel for reduced wear and tear on the motor.

This modification effectively leans out the mixture and provides more efficient combustion. Over the long run though, the PCM will use it's adaptive capabilities to make the mixture correct as read by the Oxygen sensors. But having changed the flow characteristics of the sensor, the flow across the elements is also changed, possible reduced. Thus the voltage would be lower and the effective flow would cause even leaner conditions. Also, load calculations will probably be off their original curve and injector pulses may be affected. One trick to provide for the change in flow of the no-post MAF is to make a proportional change in the sample tube that contains the sensor wires. Assuming you get the hole size just right, the transfer function of the MAF is still not likely to match that of the original function map in the computer.

The curve could be steeper sooner or flatter later or shifted completely. At a given MAF voltage the air coming into the meter is known through the transfer function table. Knowing that most people drive around in closed loop situations these variations are quenched by other inputs. Just unplug the MAF all together and you will see that it will run however poorly, but it is not dead. The problem with the sample tube issue is that the overall flow characteristics of the MAF are just different than before. If you were to compare the two curves against each other, they will be very close. And it seems for the most part that the increased airflow, especially at WOT, is of greater benefit than the slight mis calibration of the sensor.

What has to take place is the voltage should still represent a given flow, the modification just shifts the curve to provide flows at lower throttle levels. Again this can affect load. Other modifications can affect even a stock sensor. The airbox, inlet tube and intake tube to the throttle body will cause the system as a whole to be slightly different. Let's not get too freaked out though. Again, the benefits of these mods are often marginally good. One key thing to remember is that the closer the MAF is to the throttle body the higher the effect the intake feedback has on the meter's accuracy. The fewer restrictions you have between these parts also can cause problems, usually rough idle and poor performance at low throttle positions.

Ideally get the MAF farther away from the throttle body. For the greatest benefit from any modification to the MAF, its flow characteristics must be input into the transfer function table in the PCM. Recalibrating the MAF for larger injectors works somewhat but load calculations will be wrong. Most MAFs are limited by their size as to how much they can flow, but even then you can peg the electronics. Adding a supercharger can cause the meter to not function to its full potential if flows above the transfer function table range occur. The sample tube size can be adjusted to give a broader range than the .5v to 5.0v the current Ford sensors have. Below are two side by side lists of MAF transfer function for two different setups on the 1994 Mustang GT. The first number in the parenthesis is the voltage and the second is the airflow in KG/hr.

Notice that there is a maximum flow of 932 kg/hr on the first and 882 kg/hr on the second. Model year changes or different options in the system caused the difference. Also, the curve for the first one is very similar to the second all the way up to 4.0v and then the first one climbs rapidly. So even though these are the same model and year, just a MAF swap will cause a discrepancy in the way WOT is computed.

This swap is a full swap. The sensor bodies have been very standardized. Swapping just the bodies wouldn't make as much a difference as swapping the electronics. Put the first one in the second car and at the high end the car will get a voltage that represents a flow rate less than the actual flow which is higher, it will run leaner and possibly cause detonation. Now these are just examples and to specifically say what will really happen depends on the other variables. Dirty sensor elements gives less voltage at same flow. Dirty injectors may cause a lean condition as well as fuel pressure and volume.

U4P0 J4J1

# Mass Air Transfer Function
( 15.9998, 932.145 ) ( 15.9998, 882.085 )
( 5, 932.145 ) ( 5, 882.085 )
( 4.75, 808.577 ) ( 4.6001, 717.327 )
( 4.5, 697.683 ) ( 4.19995, 568.729 )
( 4.25, 598.512 ) ( 3.80005, 443.577 )
( 4, 510.114 ) ( 3.5, 362.466 )
( 3.80005, 446.745 ) ( 3.30005, 313.989 )
( 3.6001, 389.397 ) ( 3.1001, 270.265 )
( 3.3999, 337.118 ) ( 2.8999, 230.66 )
( 3.19995, 290.86 ) ( 2.69995, 195.491 )
( 3, 249.037 ) ( 2.5, 163.173 )
( 2.80005, 211.65 ) ( 2.3999, 148.281 )
( 2.6001, 178.381 ) ( 2.30005, 134.974 )
( 2.3999, 149.232 ) ( 2.19995, 122.301 )
( 2.19995, 123.568 ) ( 2.1001, 110.894 )
( 2.1001, 112.162 ) ( 2, 100.122 )
( 2, 101.389 ) ( 1.8999, 89.9828 )
( 1.8999, 91.2501 ) ( 1.80005, 80.7944 )
( 1.80005, 82.0617 ) ( 1.69995, 72.2397 )
( 1.6001, 65.2692 ) ( 1.6001, 64.3187 )
( 1.5, 57.9818 ) ( 1.5, 57.0313 )
( 1.30005, 44.9914 ) ( 1.3999, 50.3777 )
( 1.19995, 39.2882 ) ( 1.30005, 44.3577 )
( 1, 29.4662 ) ( 1.19995, 38.9714 )
( 0.75, 19.961 ) ( 1, 29.1493 )
( 0.600098, 15.2084 ) ( 0.899902, 25.0304 )
( 0.399902, 10.4557 ) ( 0.800049, 21.2283 )
( 0, 8.87154 ) ( 0.600098, 14.5747 )
( 0.5, 11.7231 )
( 0, 11.7231 )


Above Graph of '94-'95 Mustang GT To modify the MAF sensor is in all practicality a bad move without telling the computer that the flow has increased via the transfer function table. Also, modified intakes will not be as effective as presumed unless the flow changes are calculated also.

The best way to increase the flow to the motor and let it know is to get the system tested or get a system that has been flow profiled and includes the ability to program the PCM. You can't just "recalibrate" the MAF. Load will be incorrect. This can affect the durability of the motor especially when WOT driving is done. These are issues that you must take into account when debating a change to the system. There are some slight risks involved and you'll have to decide.I hope this information proves useful. Any discrepancies are my own and you can e-mail me about it. These are only the facts presented as I see them with the information I have gathered.



1968 -- Lyndon Johnson was still President, the Viet Nam War was in painful escalation, NASA was counting down to manned moon missions, ecology was becoming a big issue, and the first popular electronic fuel injection system appeared on the Volkswagen Type III fastback/squareback.

That original Robert Bosch D-Jetronic EFI used a vacuum sensor (the "D" is for "Druck," which means "pressure" in German) to inform the electronics about the intake situation, but in '74 the air flow meter showed up on L-Jetronic (the L stands for "Luftmengenmessung" auf Deutsch, meaning "air flow management"), also known as AFC (Air Flow Controlled) fuel injection. At the time, we thought the concept was ingenious: A vane rotated against spring pressure according to how much of the atmosphere the engine was ingesting, and this movement turned the shaft of a variable resistor, thus changing the reference signal before it returns to the computer. Simple, and much more accurate than anything else ever conceived.

The jump to direct mass measurement occurred in 1984 when LH-Jetronic with its hot-wire sensor was introduced (to continue our foreign language lesson, the "H" stands for "Heisz," German for "hot"). Then came the GM hot film MAF (Mass Air Flow) sensor and other configurations from such outfits as Hitachi and Mitsubishi that may be of the wire, film, or Karman-Vortex variety.

*Air sales

We've used a retail analogy before to help illustrate this evolution, but it bears repeating. Suppose you were in the business of selling air. You could calculate the amount delivered by using a complicated formula that takes pressure, temperature, and the diameter of the valve into account, but that would be slow, unwieldy, and probably not very accurate.

You'd improve the efficiency of your operation considerably if you had a way of directly measuring the volume flowing into your customers' air canisters in cubic feet per minute. That would sure speed things up, but it wouldn't be perfect, either. On cold days, they'd be getting a big bargain because any gas is denser when it's cold than when it's hot -- in essence, you'd be giving them a baker's dozen and then some. Sales would go way up (and profits way down) in winter as soon as your patrons realized that fact of physical reality. And they'd always try to buy at sea level in dry weather because altitude and humidity affect density, too.

To keep from losing your shirt, you'd still have to use a formula that varied the price per cubic foot according to temperature, altitude, and moisture content. Wouldn't it be a lot easier, faster, and more precise if you had a meter that read out in the actual weight instead of just the volume? Sure it would.

Back to engines. Speed-density EFI systems, such as you might find on your typical older Honda, use computer power to calculate the mass of intake air from input on rpm, vacuum, throttle position, and intake air temperature sensor input. But, as we said, a direct reading by means of an air flow sensor aids swiftness and accuracy, which are both critical if optimum performance, high fuel efficiency, and low emissions are to be achieved. Since air/fuel ratios are by weight (stoichiometric is 14.6 lbs. of air to one lb. of gasoline -- in gallons it would be about 2,000 to one), however, measuring mass makes even more sense, so for 15 years we've had sensors that do just that.

*Keeping it hot

Regardless of the type, a mass air flow sensor has some other advantages besides its ability to account for density: no moving parts, restrictions, or compensating sensors. A typical MAF has a wire or film element that's kept heated to a specified temperature above ambient (180 deg. F. in Bosch units) and is exposed to intake air. Through a Wheatstone bridge circuit and dedicated electronics, the amount of current required to maintain that temperature becomes the signal to the computer. High air flow obviously has a greater cooling effect than low, but so does the denser air of cold days and low altitudes, so the PCM gets the true data on mass it needs to provide the longer injector pulse width that extra oxygen needs to fire dependably. Bosch versions and GM 5.0/5.7L V8 units produce an analog voltage output, but Hitachi and most AC Delco MAF's send out a varying frequency digital signal -- a square-wave.

The Karman-Vortex air mass measurement principle first showed up on the Lexus LS 400 and Toyota Supra. The speed of vibration of a thin metal mirror in the intake vortex is monitored optically to generate a frequency signal that varies according to the mass of air entering the engine. Various Mitsubishi units use sound waves to exploit the vortex idea.


From various authorities, we've compiled quite a list of probable symptoms of MAF trouble. Bosch tells you to expect starting problems both hot and cold, hesitation, stalling (especially under load), rough idle, and low power output. GM says the engine will fire up, then die. Nissan gives stalling, poor idle, black smoke, and engagement of the fail-safe mode (perhaps limiting rpm to 2,000) as evidence of air mass measurement problems.

With all makes, contamination of the sensing element, which slows response, will result in stumble, which brings us to the most prominent logical effect of a bad or non-existent MAF signal: transient throttle glitches, including stalling, sagging, and missing. If it's far enough out of range to cause the PCM to shift to LOS (Limited Operating Strategy), overall performance and driveability will be lousy.

But don't let these symptoms cause you to jump to an unfortunate conclusion and automatically replace that expensive sensor. Plenty of other malfunctioning components can result in the same kinds of annoying and inefficient engine behavior. So, as we say over and over, check the basics first. That means ignition, compression, fuel pressure and volume, etc. A simple problem that would be embarrassing to overlook is a hole or rip in the duct between the sensor and the throttle body, which admits unmeasured or "false" air and leans out the mixture. An open PCV can do the same thing. A restricted air filter element may make trouble, too. Then there are all the other sensors and the wiring and connections that are part of every electronic engine management system. In short, don't blame the MAF right off because most of them have proved to be pretty dependable.

*On the other hand

Of course, there's still a considerable percentage of cases where the MAF is indeed the culprit. The early GM/AC MAF, for example, garnered a poor reputation for reliability. Some cars thus equipped were factory retrofitted to speed-density systems by means of a PROM change to keep reoccurrences and warranty costs within reason. By the way, the higher-frequency 10kHz Hitachi unit used on later model GM cars has a much lower failure rate.

So if the basics check out and you suspect the EFI, how do you determine if the air mass meter is shot and not some other sensor? Well, most late models have on-board diagnostics, so you might as well use them first. That's why they're there, after all, and now mandatory under OBD II regulations.


As you may have noticed, the fault codes of the car makers' proprietary self-diagnostic systems vary (as opposed to the OBD II situation), so you'll need the specific service info for the car at hand, but we'll offer a couple of examples for illustration. With a typical GM product, Code 33 means the grams-per-second signal is too high, and Code 34 means it's too low. On a garden-variety Ford EEC-IV, Code 26 tells you that the MAF (or, for that matter, the earlier VAF) is out of self-test range, Code 56 means the signal is above the max allowed, and Code 66 indicates that the signal is below the minimum.

OBD II continually examines the MAF under the comprehensive component monitor. It looks for an out-of-range signal, and compares output to values calculated from input on rpm, manifold absolute pressure, throttle position, and intake air temperature, then sets a generic DTC if there's a discrepancy.

But codes can get you into trouble. All they should really be used for is aiming your diagnostic efforts in a general direction, not as the final word on what's wrong. You'll need to do some more troubleshooting and rely on your experience before you can be sure.

*Tap test and quick check

We'll bet most of you have heard of the basic GM MAF test: When tapped with a tool (the handle of a screwdriver, not a two-pound ballpeen) at idle, a bad sensor will not only produce a dramatic change in frequency, it may also cause the engine to stumble or stall. This is certainly a convenient check, and one of MOTOR SERVICE's GM experts says it's almost 100% accurate. You might even want to try it before pulling codes.

But there's another quick check that's almost as fast. With the key off, unplug the MAF's harness connector, then start 'er up. If the engine runs appreciably better now, it's time for a new sensor.

Several service bulletins have been issued on the power and burn-off relays used with GM 5.0L/5.7L V8's. Dig them out if you're presented with a hard start, rough running, surging, or stalling complaint.

*Scan and measure

MAF diagnosis is yet another area where a scan tool works well. Errors in sensor calibration are magnified as air flow increases, so being able to test during road load can be helpful. In some cases, both the air flow value the PCM is using and the actual signal from the sensor can be displayed. If these two numbers don't match, the computer is probably reverting to a substitute value from memory because the MAF info is faulty. By the way, GM units should produce a signal that results in a reading of 4-7 grams per second at idle, or 100-240 gps at WOT (naturally, depending on the displacement and horsepower of the particular engine).

To check out a Bosch hot-wire air mass sensor, first look for battery voltage at the appropriate terminal, then measure output. A typical unit should read about 2V at idle, rising to almost 3V at 3,500 rpm. Greg McConiga, former NAPA/ASE Tech of the Year, tells us, "Diagnosing Bosch sensors can be trying, in that as little as 250 millivolts means the difference between perfection and rolling black smoke."

To give you a few more ballpark references, common output specs for Ford hot wire units are 0-0.5V key on/engine off, 0.5-1.0V at hot idle, 1.5-2.5V at hot cruise, and 3.0-4.7V at WOT. Also, you should see the voltage change when you blow air through the sensor. If you're using a lab scope (also known as a DSO for "Digital Storage Oscilloscope"), set it for two volts and 200 milliseconds per division, then tap into the sensor's signal wire. Any spikes or jagged areas in the pattern are cause for replacement.

If your DMM can measure frequency, you can use that mode to check AC, Hitachi, and any other unit you run into that produces a frequency signal. Set the meter to read Hz or kHz, and connect its leads to the sensor's signal and ground wires. An ordinary AC MAF as found on a 2.8L Chevy V6 should show you about 45 Hz at 1,000 rpm and 72 Hz at 3,500, whereas the high-frequency type of a later-model 3800 will read 2.9 kHz and 5.0kHz at those same speeds. Record the readings at various rpm and compare them to specs. You should see a linear frequency rise with no dips or jumps as speed increases.

*More better

But since these are high-frequency sensors, you might not catch a glitch with your DMM, so using a lab scope makes sense. Start out by setting amplitude to 5V per division. Changing the amplitude to one volt per division will give a different view of this signal. The timebase setting varies according to the range of the MAF. Use 0.1 milliseconds per division for a 10kHz Hitachi unit, for example. You should see a square waveform with even frequency pulses. As engine speed and load are varied, the pulse-width frequency should change smoothly and evenly. If you see any gaps in the pattern while tapping the sensor or driving the car, a new part is in order.

You can measure a Toyota/Lexus Karman-Vortex signal using a lab scope set at 1V and 10 milliseconds per division. At idle, you should get a nice even rectangular waveform. Another test, which Lexus gives in the 1990 service manual, is that of resistance compared to temperature. Unplug the sensor from the harness, then check the resistance between terminals THA and E2 of the meter's connector. At 68 deg. F., you should see 2-3 ohms. This should fall to 0.9-1.3 at 104 deg., and 0.4-0.7 at 140 deg. At the other end of the scale, 10-20 ohms is specified at -4 deg., and 4-7 ohms at 32 deg.

A final note from our friend Greg: "You might want to mention that cheap, poor quality air filters that shed fibers are causing MAF problems, particularly on GM/Hitachi units," he says. "They don't apply 'glue' to the element surface, and fibers escape and wrap around the sensing element, skewing the output."


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