A MAF sensor is a mass air flow sensor. This sensor converts air going into the engine into a voltage signal. In order to determine how much fuel needs to be injected into the cylinder, the on-board computer needs to know the amount of air going into the engine. This sensor is between the throttle body and the air cleaner directly in the line of the air intake stream. Faulty sensors will result in a decrease in engine efficiency, misfiring and other engine issues. The actual engine is fine, but a failed sensor can cause all sorts of problems.
Find the sensor specifications for your vehicle. This can be found in the user's manual for your vehicle. You can also contact the manufacturer for the specifications.
Connect the sensor scan tool to the monitor sensor valves. You will need to scan through the sensors and select the mass air flow sensor. A scan tool is handy when determining what is causing the check engine light to turn on.
Record the readings for the MAF when the car is idle. Then take readings when the car is at different RPMs. Just gently press down on the accelerator when the car is parked to change the RPM. This is easiest by having someone in the car and someone taking the sensor readings. Compare these readings to the specifications.
Take readings again by starting from idle and then gradually increasing the RPM while watching the sensor. The increase that the sensor reads should increase in proportion to the RPM increase. If any readings are out of specification, then the sensor will need replacement or repair.
Do all the tests mentioned in the above steps, but tap the sensor or heat the sensor using a hair dryer. If this causes the readings to go out of specification, then a wiring problem exists.
Monitor the MAF sensor when driving if possible. Do not drive and check the readings yourself, have someone else drive. If the problem you are worried about occurs and the MAF readings are in specification, then the MAF is not the problem.