A Subaru Legacy's second-generation On-Board Diagnostic connection is beneath the dashboard on the driver's side, to the left of the steering column. It is not hidden by an access panel.
The Subaru Legacy's OBD-II system is part of a universalised diagnostic system. The Subaru's system of trouble coding is essentially the same as what a driver would find in other vehicle brands. In 1996 the Environmental Protection Agency mandated one system for all vehicles sold in the United States. The OBD-II port, while the same in all OBD-II vehicles, will have different locations depending on both brand and model.
Data Link Connection and Scanner
The Subaru Forrester's OBD-II port is also called a Data Link Connection or DLC. The port itself connects to the Legacy's central computer, and it can be used for other diagnostic systems, like anti-lock braking or tire pressure monitoring. Accessing any of these requires a hand held diagnostic scanner.
How To Connect
Attach the diagnostic scanner to the Legacy's DLC. Most scanners will use a cable that ends in a 16-pin plug. Switch the scanner on. Turn the Legacy's engine on. Some brands of diagnostic devices may also need the engine running. Consult your device's manual for the action you wish to perform, whether it be pulling trouble codes or interacting with one of the Legacy's many sensors.