Hi! Recently bought a 2008 Subaru Forester 2.5X Premium, Non-turbo Automatic with 120,000 miles. Past couple months have had P0457 code every 50 miles or so… Changed Gas Cap to new one (non OEM tho) and still get same code. Originally had hard time starting car after fueling, so changed my Purge Control Valve behind the alternator, that solved the hard-starting post fueling, but still get the P0457 code. Fuel filler neck looks good, and no obvious hoses off or cracked. Thinking could be Charcoal Vent Canister, but do not see any cracks or loose hoses. Hooked the green connectors under the dash, and heard several audible solenoids cycling more so from front of vehicle, did not notice a clicking around the Charcoal Canister. Should I hear one from the solenoid? I removed a square cover where I think the valve/solenoid is, but not sure if that is the only one and if that should be cycling on/off. Do I need to buy a whole new canister or can I just get the cylindrical valve that was behind the square endplate? Trying to troubleshoot myself before biting the bullet and getting a Smoke Test. Any suggestions would be great, thanks!
The emission system on most vehicles is covered under warranty for an extended period, from 80 too 100,000 miles. The reasoning relates to the fact that without causing driveability problems, many owner’s will neglect to repair such a problem. Inspect the gas cap. Look for dirt on the threads, cracks and general serviceability. Install it and make sure it clicks into place. If the gas cap was to blame, the check engine light can be reset with a simple code scanner, or the code will reset after three driving cycles where the computer no longer sees the problem. Raise the vehicle and inspect all the vacuum hoses. Replace any that are cracked or have lost their flexibility and will no longer create a tight fit on the port. Check the fitting on the intake manifold for cracks at the point the emission hose connects. Check the charcoal canister for cracks. Place a vacuum pump on the emission hose (engine side) and pump it up to around 10 inches of vacuum. If there is no leak the vacuum will hold its position. If it drops, a leak is present between the engine and the purge valve on the canister.
The emission warranty on vehicles is 2 years/24,000 miles.
The only components warranted for 8 years/80,000 miles are, the catalytic converter(s), and the control computers.
Tester
This code often displays as a bad fuel cap in lookups, but it can also be set by a bad evap purge valve. If the square cover you removed was behind the rear passenger wheel, then you’ve found it.
Thank you for the replies! My car has 120,000 so out of warranty. Threads on gas cap look good, clicks and seems tight. Was looking online for a replacement Evap vapor charcoal canister valve, but it seems like I have to buy the whole canister, which is weird because the small square plate had 4 lil metal clips that came off easily, and what I believe could be the bad valve/solenoid came out easily like it was meant to be accessed and replaced. I am enclosing pics. of the cylindrical piece that came out. Is this the evap vapor valve? It came out of my charcoal canister under the rear right tire, passenger side. 
Here is a stock photo of canister that looks just like mine.