Check fuel cap message

I purchased the vehicle listed above used. I am getting "check fuel cap" message. I tightened the cap again but it did not go away. Then I got it cleared. Later on after the fill up the message came up again and now it is MIL (malfunctioning) icon on the dashboard. My mechanic has done his best but couldn't find anything. He is suggesting that may be we will have to change the sensor-is it worth it. This is fairly new car I should have to do this. Please help.