Mass Air Flow Sensor (MAF)

Automotive illustrated glossary

Mass Airflow Sensor (MAF)

Mass Airflow Sensor (MAF)

Toyota mass airflow sensor (MAF)

Volkswagen mass airflow sensor

An Air Flow Sensor or Mass Airflow Sensor (MAF) is one of the components of an electronic fuel injection system of a modern car or truck. A mass air flow sensor is usually installed inside the intake air duct between the air filter and the intake manifold of the engine.

A mass air flow sensor measures the amount of air entering the engine. Often an intake air temperature sensor (IAT) is also installed within the mass air flow sensor. There are few types of airflow sensors, however most of the 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.

A small electronic chip installed inside the airflow sensor translates the amount of this electrical current into a digital signal that is sent to the engine computer (ECM). The powertrain computer uses the air flow signal to calculate the proper amount of fuel injected into the cylinders in order to keep an optimal air/fuel ratio. This is needed to keep the exhaust emissions low without sacrificing the engine performance. In addition, the powertrain computer 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 airflow sensor problems

Problems mass air flow sensors are very common. The air flow sensor could get contaminated or damaged. Often this happens when a bad or improperly installed air filter allows some unfiltered air into the engine or when the 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 a wide range of various driveability problems such as stalling, especially when the engine is cold, misfiring, lack of power, poor acceleration, etc.
In addition, a problem with the mass air flow sensor will causes the "check engine" or "service engine soon" light to come on.

The "check engine" light comes on when the powertrain computer has detected a fault in one of the engine electronic systems. This fault is stored as a trouble code and can be retrieved from the powertrain computer with a scan tool. Following trouble codes are associated with a mass airflow 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"

The trouble codes P0171 System Too Lean (Bank 1) and P0174 System Too Lean (Bank 2) are also could be caused by a bad or contaminated mass air flow sensor. If your check engine light is on, have your vehicle's computer scanned.

Often, an air flow sensor can be checked with a scan tool. Usually, the air flow readings of the sensor in question taken at different RPMs compared to the readings of a known good sensor. Typically, a contaminated air flow sensor will show different amount of air flow than a known good sensor.

If the airflow sensor is bad, it will need to be replaced. It's a fairly simple job, however the air flow sensor itself could cost up to few hundred dollars. If the sensor is just dirty, your mechanic may suggest to clean it (cleaning the airflow sensor is a delicate procedure) as a temporary solution - sometimes this could help. If the new mass airflow sensor is installed, make sure the air filter is installed properly, as the sand particles coming with the unfiltered air can damage the airflow sensor.