The mass airflow sensor (MAF) is used to measure the amount and temperature of air that is entering the engine at any given time. The mass airflow sensor senses the total amount of air passing the sensor and allows the fuel injection system to adjust the fuel mixture to compensate for cold weather and/or high altitude conditions.
The first indicator that you may have a problem with the MAF is the presence of a check engine light (CEL) on your dashboard. The CEL can be caused by a wide variety of problems: you need to read the diagnostic trouble codes (DTC) from the computer to get a starting clue as to what the problem is. Sometimes, there is no change in how the engine is running. Other times, you might experience a loss in power, sluggish running and a decrease in gas mileage as a result.
The computer will know if something is wrong with the MAF because it will compare the values being output by the sensor to "expected" values that it should be receiving. This common-sense check by the computer helps to diagnose problems with every component in the system. If the MAF becomes dirty and is falsely indicating to the engine that the car is receiving very little air while at full throttle, then the computer will most likely trigger a DTC and light up the CEL on your dash.
To read the DTC, you'll need to access the on board diagnostic (OBD) connector under the dashboard. Through this connector, a computer is able to retrieve the codes using a diagnostic software package. Products such as the VAG-COM are invaluable for this and other diagnostic tasks. However, for those of you on a budget, many auto parts stores have a generic hand held scanner that can retrieve codes. Usually for free as well. In this case, simply write down the DTC and look it up online.
Typically, there are three DTCs that indicate a direct problem with the MAF: P0101: Mass Air Flow Sensor: Signal Implausible; P0102: Mass Air Flow Sensor: Below Limit; P0102: Mass Air Flow Sensor: Above Limit
Vacuum leaks and other air leaks in the system can also cause MAF sensor errors. If you have a crack or leak in your air intake downstream of the sensor, then the MAF will be sensing less air than the engine is actually receiving. Always check the intake system for leaks. Many times, I've encountered poor running engines that ran smooth once a hose clamp was tightened.
Cleaning the MAF involves using a can of MAF sensor cleaner (available at any auto parts store) to spray off the hooked end of the sensor. Follow the directions on the side of the can for best results. Sometimes you get lucky and the problem is just dirt buildup.
After you have replaced or cleaned the sensor and cleared the DTC with a scanner, you need to go drive the car and see if the code returns (usually around 10-20 miles). If the same error code appears, then the problem probably lies elsewhere. Most of the time when you have an error code indicating a problem with the mass air flow sensor, it is usually solved by the installation of a new sensor. However, the computer can become confused sometimes and give misleading error messages. Wire harness issues, DME problems, and vacuum leaks may all give false MAF error codes. At this point, it's best to dive into the factory manuals and start going through the laborious test procedures contained in there.
Shown here is the MAF sensor for the Mk4 Jetta 2.0 (green arrow) Replacement is a snap and should only take about 15 minutes. Use a pair of pliers to compress the hose clamp (purple arrow) and slide the intake hose off the end of the MAF sensor. Press the tab on the electrical connector down with a flathead screwdriver and pull the connector off.
Remove the upper air filter cover. See our article on Air Filter Replacement for more information. Carefully rotate the cover until you can loosen and remove the Phillips head screws (green arrows) holding the MAF sensor to the air box. Pull the MAF sensor out of the cover and fit the new one in its place, making sure that it is mounted correctly as shown here. Once in place, refit the cover, attach the intake hose and plug the electrical connector back in.