2000 Toyota Celica GT Check engine light on

Hello, my girlfriend has a 2000 Toyota Celica GT, and the check engine light came on a few months ago. We had taken it to a shop, and they told us it was the fuel cap thats leaking pressure even though there r no visible damage to the stock fuel cap. So they replaced it and reset the light. After about another few hundreds miles of driving the light came back on again. This time we took it to a mechanic that's recommended by a relative and he too can not figure out what the problem is and the diagnostic code said "evac system". He said he could not find any leaks, and kept on asking us if we had not tighten the fuel cap after we fill up. We had not had the chance to fill again after the check engine light was reset last time. So he put on the fuel cap, reset the light again and told us to drive around and see what happens. A few days later, we went to fill up the gas and I made sure that cap was tightened where you hear the clicking sound. A few miles down the road, the check engine light came back on again! The car drives normal and nothing that we can feel are unusual when driving. Please advise. Thank you!