Get a new garage.

There are many reasons the light can come on then go out again, most of them
have to do with the gas cap. Most of the reasons that the light can come on
require service activity to correct and then turn the light off. But the
evaporative emissions (gas fumes) control system has a huge hole in it where
you put the fuel nozzle. The gas cap is supposed to seal, and when it does
then the System detects the appropriate vacuum or pressure -- I forget
which. In any case, the gas cap itself is a variable that can cause the
light to come on and go off.

AutoZone will let you use the Code Reader Tool to pull the codes down and
reset the computer.