When the Forester battery cables are disconnected, it clears the memory of the cars computer, when reconnected, the computer alarm will go off, Look for a small push button switch just to the left side of steering wheel, and just under the lower dash.. turn your car on to the assoc. position, or start it, then push the little bottom under dash to reset the alarm.
The complete system, gas regulation, etc. also needs to be reset after battery has been disconnected.. Start car and with foot on bake, and emergency brake set, put car in N then into Drive and let the car warm up sitting there, once showing the needle to have moved to start warming up, turn on air conditioner to let the computer reset RPM for the air conditioner.. That's it.. the computer should have reset all values of sensors and operations.
Note.. If anyone has changed their own timing belt w/ gear pulleys & tentioner, also needs to know this procedure for resetting the computer before car will run correctly.
Hope this helps...