Check The Mass Air Flow Sensor Unit

Connect the battery positive terminal to terminal No. 1 and the battery ground terminal to terminal No. 2, the circuit tester positive terminal to terminal No. 3 and the circuit tester ground terminal to terminal No. 2.

Fig 1: Connecting Battery Positive Terminal To Terminal No. 1 And Battery Ground Terminal To Terminal No. 2
Courtesy of SUBARU OF AMERICA, INC.
Check the voltage changes when air is blown to the mass air flow sensor unit in arrow direction.

Fig 2: Checking Voltage Changes When Air Blown To Mass Air Flow Sensor Unit

Courtesy of SUBARU OF AMERICA, INC.