I replaced the Oxygen sensor and the error code went away. Was feeling smug about finally not having any "Check Engine" lights, started on a trip and after about 200 miles "Check engine" again. This time it is P0445 Large leak in evap system. Clicked gas cap a couple of times, cleared the code, and it stayed clear for another 200 miles or so, then came back. checked gas cap again, cleared code, got back on the highway, then i get a warning on the message center "gas cap loose" or something to that effect. Clicked the gas cap again, the warning went away but a few miles later the Check Engine is back. Where else can this large leak be?