What Does the Mass Air Flow Sensor Do?
MAF sensors, aka Mass Airflow Sensors, get their name from the fact that they measure the mass, and hence, the volume of air that enters the engine. This value is arguably the most important parameter all engine management systems use to calculate fuel delivery strategies.
Why is the Mass Air Flow Sensor Needed?
Since the volume of air per any given unit of measurement is directly linked to the density of the volume of air, all engine management systems need real-time input data on the amount of air that enters the engine through the intake system.
However, the density of atmospheric air is also greatly affected by things like relative humidity, elevation above sea level, and ambient temperature. In practice, since this means that the volume of air that enters the engine changes almost continuously, all engine management systems can adapt the volume of fuel the injectors deliver to the cylinders to suit the volume of air that enters the engine at all times.
Thus, even if the density, and by extension, the volume of air that enters an engine changes significantly, the MAF sensor will measure the airflow through it, and report the volume of air passing through it at any given moment to the engine management system. The PCM will then adapt the volume of fuel being delivered to the cylinders (via the injectors) to suit the volume of air that is available at that moment.
In this way, the PCM can maintain an air/fuel mixture (for gasoline) that is as close to the ideal proportions of one part of fuel to every 14.7 parts of air as possible. These proportions are known as a stoichiometric air/fuel mixture, but it should be noted that since normal driving sometimes demands more fuel relative to air than at other times, the stoichiometric proportions of the air/fuel mixture is the “starting point” from which engine management systems calculate fuel delivery strategies.
Put differently, engine management systems cannot calculate appropriate fuel delivery strategies unless they have accurate, real-time input data about the volume of air that enters the engine at any given moment. Therefore, MAF sensors play critically important roles in the overall management of the air/fuel mixture to increase engine performance while lowering harmful exhaust emissions at the same time.
How Does the Mass Air Flow Sensor Work?
Almost all modern MAF sensors are of the hot-wire type. In their simplest form, hot-wire MAF sensors use a piece of wire made of platinum to infer the volume of air that flows over it; here is how this works-
In all modern MAF sensor designs, the actual sensor that contains the length of platinum wire is placed directly in the inlet duct in such a manner that it is in direct contact with the air flowing through the inlet duct. To make the sensor work, the wire is connected to the PCM via dedicated circuits; the PCM supplies a small electrical current to the wire, which acts like a heater element when the current flows through it.
While specifications differ between vehicle makes and models, most hot-wire sensors will heat up to about 200 degrees F, and if the engine is running at idle the low flow rate of air through the intake system will cool down the wire. This cooling effect is measured by a dedicated temperature sensor, but at idling speed, the cooling effect will be relatively small. However, as the engine speed increases, the higher airflow will have a progressively greater cooling effect on the wire, but since the PCM “knows” how much current it used to heat the wire to 200 degrees, it can infer the volume of the air passing over the wire based on the additional current required to re-heat the wire back up to 200 degrees.
Put differently, temperature changes in the wire element caused by changes in the airflow over the wire element produce changes in the electrical resistance of the wire element, which changes the PCM interprets as changes in airflow through the intake tract. This information is, however, not the only data the PCM uses to adapt the volume of fuel the injectors deliver to the cylinders; the PCM also considers input data from other engine sensors, which among others, include the-
- engine coolant temperature sensor
- barometric pressure sensor
- intake air temperature sensor
- throttle position sensor
- throttle pedal position sensor (if fitted)
- engine speed sensor
- camshaft position sensor(s) on engines with variable valve or camshaft timing
As a practical matter, the PCM cannot achieve or maintain a stoichiometric air/fuel mixture without input data from the sensors listed above. Thus, while the MAF sensor plays a critical role in fuel management, input data supplied by the MAF sensor represents only one aspect of a complex interaction between many other sensors.
NOTE: See the section on “Symptoms” for more details on why some engines continue to run even if the MAF sensor is defective or has stopped working.
Where is the Mass Air Flow Sensor Located on the Engine?
This image shows the location in the intake tract on a Toyota Corolla. In this example, the two red arrows indicate two of the four small bolts that keep the MAF sensor attached directly to the throttle body, while the yellow arrow indicates the clamp that seals the intake tract where the MAF sensor fits into the intake tract. The green arrow indicates the actual MAF sensor where it fits into the larger plastic housing. In almost all cases, the actual sensor is removable from the plastic housing for cleaning, servicing, or replacement purposes.
Note that in all cases, the MAF sensor will always be located in the intake tract as shown here, but sometimes, the MAF sensor housing may not attach directly to the throttle body. In these designs, the MAF sensor will be secured in the intake tract by two hose clamps to facilitate the removal of the MAF sensor housing from the intake tract.
What Does the Mass Air Flow Sensor Look Like?
This image shows an example of a MAF sensor that is representative of all MAF sensor designs. As a result of all MAF sensor designs following the same general pattern, it is always easy to locate and identify MAF sensors in the inlet tracts on all applications that use these sensors.
What are the Symptoms that the Mass Air Flow Sensor is Bad?
SPECIAL NOTES: Although the most common symptoms of defective or malfunctioning MAF sensors are largely similar across all applications that use MAF sensors, in some cases, a defective or malfunctioning MAF sensor will not produce noticeable drivability issues.
Depending on the vehicle make, some engine management systems will revert to a default fuelling strategy when the MAF sensor fails or stops working. As a practical matter, default fuelling strategies are typically set to run rich, meaning that there will be more than one part of fuel for every 14.7 parts of air in the air/fuel mixture. Moreover, default fuelling strategies also do not allow the PCM to adapt the injectors’ “ON” time in any way, meaning that the fuel delivery system runs in open-loop mode under all operating conditions.
In technical terms, “open-loop” operation refers to a condition in which the fuel delivery system does not react to inputs from any sensors or devices that would normally be involved with regulating the volume of fuel the injectors inject into the cylinders. By way of contrast, “closed-loop” operation refers to a condition in which the fuel delivery system reacts to inputs from sensors and other devices to control the fuel delivery system based on inputs from said sensors and devices.
As a practical matter, adopting a default fuelling strategy allows the engine to continue operating after the MAF sensor has failed or stopped working, and in most cases, the adoption of a default fuelling strategy will not produce significant or even noticeable drivability problems. Note, though, that fuel economy will decrease markedly while the default fuelling strategy is in operation. END OF SPECIAL NOTES.
As a general rule of thumb, the most common symptoms of failed or malfunctioning MAF sensors are largely similar across all applications but as mentioned above, some vehicles may produce only a stored trouble code and an illuminated warning light as a symptom of a malfunctioning or defective MAF sensor. Nonetheless, some common symptoms of failed or defective MAF sensors could include one or more of the following-
- Stored trouble code and illuminated warning light
- In some cases, multiple trouble codes may be present, especially misfire and/or fuel trim-related codes
- The engine may hesitate or stumble upon acceleration
- Fuel consumption may increase dramatically
- The idling quality may be poor, or the engine may not run at idle at all
- The engine may exhibit misfire-like symptoms at some or all engine speeds and loads
- Depending on the vehicle and the nature of the problem, the vehicle may emit visible black smoke from the tailpipe at all or some engine speeds
- Depending on the vehicle and the nature of the problem, it may be difficult to start the engine
- Potentially fatal damage to oxygen sensors and/or catalytic converters may occur if the MAF sensor issue persists over an extended period
NOTE: Depending on the vehicle make and model, all or most of the symptoms listed above can also be caused by engine vacuum leaks, poor quality fuel, and/or a dirty/clogged air filter element.
How do you test the Mass Air Flow Sensor?
Note that while it is possible to test a MAF sensor by measuring the electrical resistance across the wire element, most MAF sensors operate in such a wide range of allowable resistance values that such tests often yield inconclusive or even misleading results.
However, since MAF sensor elements are located directly in the intake air stream, the wire elements are often affected by a layer of dust, oil residues, or other forms of contamination. Therefore, remove and clean the element with an approved MAF sensor cleaner- without touching the element with your hands or any other implement. Touching the wire element will almost certainly damage the special coating that protects it, so don’t touch the MAF sensor’s wire element with anything.
In nine cases out of every ten, cleaning the MAF sensor element will restore its functionality, so consider cleaning the sensor’s element as a kind of screening test to assess the operation of a suspect MAF sensor.
NOTE: If you have to replace the MAF sensor, be aware that almost all German and (some) Japanese vehicles do not tolerate aftermarket MAF sensors. Therefore, we highly recommend that you only replace defective MAF sensors with OEM parts on German or Japanese vehicles to avoid replicating or aggravating the problem you are trying to resolve.
How do you replace the Mass Air Flow Sensor?
In most cases, replacing a MAF sensor is no more complicated than disconnecting the sensor’s electrical connector and undoing the hose clamps or retaining bolts that secure it to the throttle body, and extracting the sensor’s body from the flexible intake ducting.
Installing a new sensor is done in the reverse order of removal, but pay special attention to the arrow or other markings that indicate the airflow’s direction through the sensor. Installing the sensor the wrong way around will cause it not to work, so bear in mind that the wire element must face into the airflow as seen from the air filter element toward the throttle body.
Be sure to tighten all hose clamps and/or retaining bolts, screws/nuts properly and double-check that the sensor’s electrical connector engages positively with the sensor. Note also that, MAF sensors are “plug-and-play” components, so the new sensor does not have to be programmed or integrated into the engine management system since this function is performed automatically by the PCM during the first few drive cycles after the MAF replacement.