I fianlly got my car back to my friend with the analyzer and the only code showing was indeed P0638 “Throttle Actuator Control Range / Performance (Bank 1).” I googled that text directly and there’s a ton of discussions about the problem. I’ll need to take some time to review them. One annoying aspect of what I looked at so far is one thread that presents a diagnostic approach for the code. http://legacygt.com/forums/showthread.php/help-p0638-138200.html?t=138200
The dealer never mentioned trying to diagnose, but went directly to guessing about replacing whole components, like the throttle body and ECM. Maybe I’ll take it up with Subaru America.