The P0457 CEL (check engine light) isn't actually documented in my 2002 Subaru WRX factory repair manual. Luckily, Google does list it ;)
The problem is "Evaporative Emissions Control System - [Gross] Leak Detected"
The good news is that it's likely one of two things.
1. Your gas cap isn't on tight enough
-Or if you're like me and just installed a new turbo inlet host (Perrin!) it's probably
2. The EVAP purge hose.
Here is what I'm talking about. Look at the blue nozzle that isn't connected to anything. It's down under the intake manifold to the left of the alternator.
The little hose down and to the right of the blue nozzle is supposed to be connected. I must have yanked it off while changing the intake.
The picture below is what it's supposed to look like when the hose is hooked up properly. If you snapped the connector (I didn't) you'll need to buy a new part.