On older model Subes you can retrieve codes using the Check Engine Light on
the dashboard.

Even when the check engine light isn't on you may still have trouble codes
stored in the computer.

Under the dash, usually between the steering column and gas pedal you should
find some harnesses that are not plugged in.  A green set, and a single
wire, black set.  If you plug the two, single wire black harness halves
together, then turn your ignition key to the run position, your check engine
light should flash out any stored codes.  Long flashes indicate the ten's
place, short flashes the one's place.  So, long-long-short, would indicate a
code 21.  (-;  Codes will be in numerical order, from lowest to highest,
separated by a pause in the flashing, and after the last code is displayed,
the codes will be listed from the beginning again.

More in depth instructions and trouble code descriptions can be found here:

http://www.troublecodes.net/Subaru/

~Brian