I had the same problem. In my case, I drive very fast over bad roads and what happened is the charcoal canister broke internally causing charcoal to plug the vent line from the tank. pulled off the vent line, got all the charcoal out of it and replaced the canister. all good. btw, mine broke 2 times, 3 in total between my 2 Subarus. that was the only failure my cars had and I am a bit abusive. love my subaru.