What It Does: The Main Relay in your Subaru is triggered by the ECU to send power to the car. It sends power to turn on the ECU, injectors, coils, MAF, boost controller, etc. It's called the main relay because it basically powers all of the engine related functions.
Cars It Works On: All Subaru models built in 1991 to 2007 including WRX, STI, EJ Engines built in 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, and 2007.
How to Know if you Have an Issue: You can tell your main relay needs replacing if the ECU won't turn on but the fuse is good.
Where is it Located in my Subaru?:
1993-2001 models the main relay is located on the driver side wall under the dash (see above picture).
2002-2007 models the main relay is located on the passenger side by the door.