Mass Air Flow Sensor (MAF)

Updated: June 25, 2017

A Mass Airflow Sensor (MAF) is one of the components of a car electronic fuel injection system. A mass air flow sensor is installed between the air filter and the intake manifold of the engine.

Mass Airflow Sensor (MAF)
Mass Airflow Sensor (MAF)

The main job of the mass air flow sensor is to measure the amount of air entering the engine or the air flow. Often an intake air temperature sensor (IAT) is also installed within the mass air flow sensor. There are few types of air flow sensors, however, most 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 very little air flows pass the hot wire, so it takes very small electrical current to keep the wire hot. When you press the gas, the throttle opens allowing more air to 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 amount of this electrical current is proportional to the amount of the air flow.

Mass Airflow Sensor (MAF)
Toyota mass airflow sensor (MAF)
Mass Airflow Sensor (MAF)
Volkswagen mass airflow sensor

A small electronic chip installed inside the air flow sensor translates the amount of this electrical current into a digital signal that is sent 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 mass air flow sensors are very common. The MAF sensor could get contaminated or damaged. Often this happens when an old or improperly installed air filter allows some unfiltered air into the engine or when oil from an oil-soaked air filter contaminates the air flow sensor.

A bad or contaminated mass air flow sensor cannot measure the amount of air flow correctly. This can cause various driveability problems, including no-start, stalling, lack of power and 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 alter the shifting pattern of the automatic transmission.

Read about testing and replacing the air flow sensor on the next page.