The air flow meter, or mass air flow sensor, measures the amount of air travelling through the air intake system. Since this air will eventually make its way into the engine's combustion chamber, the meter needs to be able to relay important information about how much air is flowing into the fuel injection and engine chambers. If your vehicle starts to idle rough, has problems starting or frequently stalls during normal driving, you may have a faulty or failing air flow meter. It may need to be replaced, but before doing so, it's a good idea to check it once you have removed it from your vehicle, since this component can be very expensive.
Inspect the air flow meter sensor for any dirt or dust. Any debris on the sensor wires can prevent the sensor from functioning properly. The sensor looks like a "T". To check the sensor wires, inspect the lower half of the "T". The sensor wires will be on the bottom of the sensor.
Set your voltmeter to the ohms setting by turning the dial on your meter to the "Ω" on your meter.
Measure the resistance between terminals THA and E2. Place one of the voltmeter's leads on the THA terminal. Place the other lead on the E2 terminal. It does not matter which leads are placed on which terminal, since ohms measures resistance. The reading should be at or near 0. If it is, move on to Step 4. If it is not, your air flow meter is faulty.
Turn the ignition switch to the "II" position.
Set the voltmeter to "volts" and connect the positive lead (the red lead on your voltmeter) to the VG terminal and the negative lead (the black lead) to the E2G terminal.
Blow into the sensor portion of the air flow meter (try not to spit on the meter or the wires) and check the voltage simultaneously. It should fluctuate as you blow on the meter. If it fluctuates, then your mass air flow sensor is working properly. If the voltage does not fluctuate, then your air flow meter is faulty.
For specific information about checking the air flow meter on your vehicle, consult the particular vehicle's manual (see Resources).