2006 Chevy Cobalt, check engine light keeps coming on

She has about 250,000k on her, and as I was preparing to take her for an e-test the check engine light came on. I brought her in to be serviced and my mechanic advised me it was the fuel and evap lines, the connections had rusted out and needed to be replaced. I have heard this is a common job on older cars. He replaced the lines and connections, light went off, I drove for a week and about 200km when the light came back on. I brought her back in and the same code came back: Evap. He cleared the code, tightened the gas cap, and said to try again.
This time I drove for about 100k, light stayed off, he checked the codes and it came back clear, and I passed the e-test.
The next morning, damn check engine light is back on.
Is this going to keep happening? Do I need to be worried?