Mass Air flow Sensor (MAF): how it works, symptoms, problems, testing

Updated: December 15, 2018

A Mass Air flow Sensor (MAF) is one of the components of a car electronic fuel injection system. It is installed between the air filter and the intake manifold of the engine. The mass air flow sensor measures the amount of air entering the engine or the air flow.

Mass Air flow Sensor (MAF)
Mass Air flow Sensor (MAF): how it works, symptoms, problems, testing

In modern cars, the intake air temperature sensor (IAT) is also installed within the mass air flow sensor. There are few types of air flow sensors, however, modern cars use a hot-wire type. Let's see how it works.

How a hot-wire air flow sensor works

A hot-wire mass air flow sensor has a small electrically heated wire (hot wire) and a small temperature sensor installed close to the hot wire. The temperature sensor measures the temperature of the air near the hot wire.

When the engine is idling, a small amount of air flows around the hot wire, so it takes a very low electric current to keep the wire hot. When you press the gas, the throttle opens allowing more air flow over the hot wire. The passing air cools the hot wire down; the more air flows over the wire, the more electrical current is needed to keep it hot. The electric current is proportional to the amount of the air flow.

Toyota Mass Air flow Sensor (MAF)
Toyota mass airflow sensor (MAF)
Mass Air flow Sensor (MAF)
Volkswagen mass airflow sensor

A small electronic chip installed inside the air flow sensor translates the electrical current into a digital signal and sends it to the engine computer (PCM).

The PCM uses the air flow signal to calculate the proper amount of fuel injected into the cylinders in order to keep the air/fuel ratio at the optimal level. This is needed to keep the exhaust emissions low without sacrificing the engine performance. In addition, the PCM uses the air flow readings to determine the shift points of the automatic transmission; so if the air flow sensor doesn't work properly, the automatic transmission might shift gears differently.

Mass air flow sensor problems

Problems with mass air flow sensors are common in many cars, including BMW, GM, Mazda, Toyota, Nissan and some other brands. The sensor element could get contaminated or damaged. Sometimes, an improperly installed or ripped air filter can cause the MAF sensor to fail sooner. Over-soaking a washable air filter with oil can also cause troubles with the air flow sensor.

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Symptoms of a bad mass air flow sensor

A contaminated or failed mass air flow sensor cannot measure the amount of air flow correctly. This causes the engine computer to miscalculate the amount of fuel to be injected. As a result, a failed mass air flow sensor causes various driveability problems, including a no-start, stalling, lack of power, poor acceleration. In other cases, a car may have poor gas mileage and black smoke from exhaust. In addition, a faulty mass air flow sensor will causes the Check Engine or Service Engine Soon light to come on. In many cars the air flow sensor signal is used to determine automatic transmission shifting points. A problem with the airflow sensor could also alter the shifting pattern of the automatic transmission.

Mass Air flow Sensor (MAF)
Mass Airflow Sensor (MAF)

When the air flow sensor signal vary from the expected range, the engine computer detects a fault and stores the corresponding fault code, illuminating the Check Engine light on the dash. This fault or trouble code can be retrieved with a scan tool. Following trouble codes are usually associated with a mass air flow sensor:
P0100 - Mass Air Flow Circuit Malfunction
P0101 - Mass Air Flow Circuit Range/Performance
P0102 - Mass Air Flow Circuit Low
P0103 - "Mass Air Flow Circuit High"
P0104 - "Mass Air Flow Circuit Intermittent"
Trouble codes P0171 System Too Lean (Bank 1) and P0174 System Too Lean (Bank 2) are also often caused by a bad or contaminated mass air flow sensor.

How the mass air flow sensor is tested

In some older cars an air flow sensor can be tested with a voltmeter. This procedure involves measuring signal voltage at different engine speed and comparing it to specified range found in the service manual.

In modern cars, the only way to test the mass air flow sensor is with a scan tool. Mechanics measure the mass air flow sensor readings at different RPMs with a scan tool. They compare the readings the specifications or to the reading of a known-good mass airflow sensor.

Testing Mass Air flow Sensor (MAF)
Mass Airflow Sensor (MAF) readings of a 2.4L 4-cylinder engine at different RPMs

Often the mass air flow sensor readings are measured at idle, 1,000 RPM, 2,000 RPM and 3,000 RPM. A contaminated or bad air flow sensor will, in most cases, show lower air flow readings than a known good one. In some rare cases a bad sensor may show higher readings. Of course, different engines will have different readings. The number mostly depends on the engine volume, so a V6 or V8 engine will have higher readings.

Low mass air flow readings do not mean the sensor is bad. If an air filter is completely dirty or if the catalytic converter is clogged up, the air flow sensor readings will be lower. Vacuum leaks may also cause lower sensor readings. That's why mechanics use a known-good sensor to compare. Read more about vacuum leaks.

Is there a way to check the mass airflow sensor readings at home? Of course, for example, here we used the free Torque app to measure the mass air flow sensor readings at different RPMs. This mass air flow sensor is good. To use any phone app that connects to your car, you will need a small Bluetooth adaptor that plugs into the OBD connector. We also posted several links at the bottom of this article, where you can get a subscription-fee based access to the factory service manual.

Sometimes poor electrical connection at the air flow sensor connector could also cause the air flow readings to be out of range. For this reason, the air flow sensor connector terminals as well as the wiring needs to be carefully inspected.

Debris inside the Mass Air flow Sensor (MAF)
In this car, stalling was caused by a piece of a leaf stuck inside the MAF sensor

Often if an air filter is not installed properly, or the air filter box is not closed, a piece of debris can get sucked in into the mass air flow sensor (see the photo) and cause problems. Sometimes the debris can fall in during the air filter replacement. In this case, the repair is easy. The mass air flow sensor must be cleaned and the air filter must be reinstalled correctly or replaced.

Air flow sensor replacement

If the air flow sensor is bad, it will need to be replaced. It's a fairly simple job. The part could cost from $89 to $380. If the sensor is just dirty, your mechanic may suggest cleaning it (cleaning the air flow sensor is a delicate procedure) as a temporary solution; sometimes it could help. If the new mass air flow sensor is installed, make sure the air filter is installed properly, as the sand particles coming with the unfiltered air can damage the air flow sensor.

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