sometimes when I fillup the gas tank, the service engine comes on,

You obviously shouldn't have to open and close the gas cap. What codes are stored? If an evap code ,the computer only tests the evap system under certain criteria (fuel tank fuel level between approx 1/4 and 3/4 full). Gas caps are cheap you could try that. The computer takes a while to recognize that the cap has been replaced and when it does see the fault is gone it will erase the Check Engine Light itself but it may take two weeks of driving or more. If a scan tool is used make sure codes stored are written down and you have a copy of the codes.