This may seem like a silly question, but have you checked your gas cap? Is it the stock gas cap, or aftermarket? Reason I ask is I bought my truck brand spankin new from the dealership with what I thought at the time was a "stock" locking gas cap. Check engine light came on once.. they replaced the charcoal canister in the gas tank. Then it comes on again about two days later, and they tell me (at a different dealership) that my gas cap is an aftermarket gas cap and was not sealing correctly, causing me to lose fuel system pressure and cause my check engine light problem. They also inform me that Toyota does not even MAKE an OEM locking gas cap. So... 30 dollars for a new gas cap, problem fixed. I just coughed up the money 'cause I had neither the time nor desire to fight with the dealership I bought it from to replace it for free.