I always close gas cap properly. I have heard "air pressure" sound coming from gas tank when filling the tank. Warnings come on typically when have driven for awhile. Sometimes it goes off when I fill the gas tank. Took it to Toyota dealer. They said it was code p0012. Changed oil etc and said maybe it was just low oil or dirty oil. Seemed all fixed. But came back two days later. Driving long distance 3-4 times a week (four hours each time). REALLY afraid of getting in black hole of repeat repairs that do not stop the problem! Minivan has nearly 130,000 miles. I'm not noticing any performance issues.