Subaru P0102 — diagnose and fix
Subaru P0102 trouble code is defined as P0102 Mass or Volume Air Flow Circuit Low Input. This refers to the Mass Airflow Sensor (MAF) or simply Air Flow Sensor (Subaru definition). This is a combination airflow and air temperature sensor. See the diagram below.
How Subaru MAF Flow sensor works
Power comes into the MAF on terminal #1 from the power relay. The sensor receives ground at the ECM B135 connector, terminal 30. The MAF receives a reference voltage on terminal 2 from the ECM B135 connector, terminal 34. Based on airflow, it reports .3-4.5-volts back from MAF terminal 3 to the ECM at the B135 connector, terminal 26. This signal wire is surrounded by a braided shield that connects to the ECM at B135 connector, terminal 35. The shield wire is the cause of many problems on this vehicle (see below).
Causes of Subaru P0102
• MAF failure
• Short to ground on MAF signal wire. Disconnect the MAF connector and ECM B135 connector. Check the resistance between B135, terminal 26, and chassis ground. The result should be less than 1Ω. If not, check for short to ground between ECM and MAF.
If any of the braided shielding wires touch either terminal 3 at the MAF or terminal 26 at the ECM, you can have a no start or stall condition.
• Open on MAF signal wire. Disconnect connect at MAF and B135 at ECM. Check resistance between B135, terminal 26 and MAF 3. The result should be less than 1Ω. If not, repair open between the ECM, terminal 26 and MAF 3.
• Check for good terminal contact at MAF and ECM B135
©, 2017 Rick Muscoplat