DTC P0102 Mass Or Volume Air Flow Sensor "A" Circuit Low: Notes

DTC detecting condition: 

Immediately at fault recognition

Trouble symptom: 

• Improper idling
• Engine stalls.
• Poor driving performance
  CAUTION:

  • After servicing or replacing faulty parts, perform Clear Memory Ref. to ENGINE (DIAGNOSTICS) (H4DOTC)>Clear Memory Mode . and Inspection Mode Ref. to ENGINE (DIAGNOSTICS) (H4DOTC)>Inspection Mode .
  • Use the check board when measuring the ECM terminal voltage and resistance.

Wiring diagram: 

Engine Electrical System Ref. to WIRING SYSTEM>Engine Electrical System>WIRING DIAGRAM .

1. CHECK CURRENT DATA. 
   1. Start the engine.
   2. Using the Subaru Select Monitor, read the value in [Air Flow Sensor Voltage].
      NOTE: For detailed procedures, refer to "Data Monitor". Ref. to ENGINE (DIAGNOSTICS) (H4DOTC)>Subaru Select Monitor .

   Is the value of [Air Flow Sensor Voltage] less than 0.2 V?

   • Yes  : Go to 2. 
   • No  : Even if DTC is detected, the circuit has returned to a normal condition at this time. Reproduce the failure, and then perform the diagnosis.
     NOTE: In this case, temporary poor contact of connector, temporary open or short circuit of harness may be the cause.
2. CHECK POWER SUPPLY OF MASS AIR FLOW AND INTAKE AIR TEMPERATURE SENSOR. 
   1. Turn the ignition switch to OFF.
   2. Disconnect the connectors from the mass air flow and intake air temperature sensor.
   3. Turn the ignition switch to ON.
   4. Measure the voltage between mass air flow and intake air temperature sensor connector and engine ground.

      Connector & terminal 

      (B3) No. 1 (+) - Engine ground (-):

   Is the voltage 10 V or more?

   • Yes  : Go to 3. 
   • No  : repair the harness and connector.
     NOTE: In this case, repair the following item:

     • Open circuit in harness between main relay and mass air flow and intake air temperature sensor connector
     • Poor contact of main relay connector
3. CHECK HARNESS BETWEEN ECM AND MASS AIR FLOW AND INTAKE AIR TEMPERATURE SENSOR CONNECTOR. 
   1. Turn the ignition switch to OFF.
   2. Disconnect the connector from ECM.
   3. Measure the resistance of harness between ECM and the mass air flow and intake air temperature sensor connector.

      Connector & terminal 

      (B134) No. 16 - (B3) No. 3:

   Is the resistance less than 1 Ω?

   • Yes  : Go to 4. 
   • No  : Repair the open circuit in harness between ECM and the mass air flow and intake air temperature sensor connector.
4. CHECK HARNESS BETWEEN ECM AND MASS AIR FLOW AND INTAKE AIR TEMPERATURE SENSOR CONNECTOR. 

   Measure the resistance between ECM and engine ground.

   Connector & terminal 

   (B134) No. 16 - Engine ground:

   Is the resistance 1 MΩ or more?

   • Yes  : Go to 5. 
   • No  : Repair the short circuit to ground in harness between ECM and mass air flow and intake air temperature sensor connector.
5. CHECK FOR POOR CONTACT. 

   Check for poor contact of ECM and mass air flow and intake air temperature sensor connector.

   Is there poor contact of ECM or mass air flow and intake air temperature sensor connector?

   • Yes  : Repair the poor contact of ECM or mass air flow and intake air temperature sensor connector.
   • No  : Replace the mass air flow and intake air temperature sensor. Ref. to FUEL INJECTION (FUEL SYSTEMS) (H4DOTC)>Mass Air Flow and Intake Air Temperature Sensor .