Son,s stock 03 WRX has a P0171 code
 

His diagnostic tool says "too lean bank one".
Is bank one on the passenger side or driver's side? I suspect an air leak. My CD for the WRX manuals is packed away somewhere or I would look it up myself.

Bank 1 is drivers side, but the problem is, since the exhaust comes down to only one air/fuel ratio sensor, it can't really determine which is which. It can only tell you that it's reading lean. "Bank one" is just how the computer's set up to display codes, but that doesn't mean that it's only the left bank. It's possible there's a vacuum leak, or lack of fuel (sticky injector?) It's also possible that the AFR sensor has simply gone bad...

The leak trick is usually pretty easy: Start the car, and spray carb or brake cleaner around the intake manifold and such. If the idle jumps up, there's your leak.

Cory, that you? Thanks for the info. Does a clogged fuel filter cause a lean condition sometimes? The car has 56k miles. Are sticky fuel injectors a common problem at 56k?

Yep, it's me! Yeah, a clogged filter can cause that too. I didn't want to bore you with way too many details, but pretty much anything that will cause the engine to not get enough fuel, or too much air, can cause a lean condition. Anything in the fuel system i.e.: pump, lines, filter, rails, injectors, pressure regualtor. OR, an air or vacuum leak after the air flow meter, thus without the airflow meter being able to tell the ECU exactly how much air is going in, and causing the lean condition.