Well Eightdollar, after following all the subaru or any manufacturer\'s books right through, when every component including the wiring has been checked, we often find they say to fit a "known good unit". Generally that\'s only in the case of ecus though.. most other components there are specific tests for, and paths to follow to arrive at the conclusion that you should check that component.
That said, once cars have been modified, or played around with, then it rapidly becomes a guess.
I\'m a mechanic, I don\'t get paid for guesses that aren\'t right, or that are only half the fault, the victim of some other component failing... I have to be methodical, systematic, and careful, whether the car is modified or not, whether I know it\'s history or not, whether the fault seems obvious or not. I always start in much the same way.. In the long run, over time, experience tells us that is the surest way.
And we know how many thousands of dollars get wasted in here fitting parts on a guess...