Mass Airflow Sensors (MAF) have the responsibility of reporting the amount of air entering the engine to the Powertrain Control Module (PCM). The PCM also uses this input to calculate the amount of engine load.
There are several designs of MAF sensors, but the most common, by far, is the hot wire MAF sensor. The hot wire MAF has two sensing wires. One wire is heated and the other is not. The microprocessor (computer) inside the MAF determines the amount of air entering the engine by how much current is required to keep the hot wire about 200 ℉ hotter than the cold wire. Whenever the temperature difference between the two sensing wires changes, the MAF will either increase or decrease the current to the heated wire. This equates to more air into the engine or less air into the engine.
There are a number of driveability issues that arise from faulty MAF sensors.
1. Runs rich at idle or lean under load
These symptoms indicate a MAF that has a contaminated hot wire. The contamination comes in the form of spider webs, potting compound from the MAF sensor itself, dirt that gets attached to oil on the MAF from an over-oiled aftermarket air filter, and more. Anything that acts like insulation to the hot wire will cause this type of issue. The fix is as simple as cleaning the MAF sensor with an approved cleaner, which the technicians at YourMechanic can do for you if they determine that this is your root problem.
2. Constantly runs rich or lean
A MAF sensor that continuously over-reports or under-reports airflow into the engine will cause an engine to run rich or lean. If the engine control system is working correctly, you will probably never notice this except for a change in fuel economy. A trained technician will have to look at fuel trim status with the use of a scan tool to verify this. A MAF sensor that behaves like this will require replacement. Before replacing the sensor, however, the rest of the circuit should be checked for proper operation. If the circuit (wire) has an issue, replacing the sensor will not solve your problem.
3. Rough idle or stalls
A completely failed MAF sensor will not send any airflow information to the PCM. This prevents the PCM from accurately controlling fuel, which will result in an engine that either will idle roughly or not at all. Obviously, replacing the MAF sensor is needed in this cases.