Check engine code

You did not list a fault code number, but for most vehicles those codes only point to a "small leak detected" or a "gross leak detected". The most common cause of a gross leak is a loose gas cap, but it is not the only cause. Because most leaks in the fuel vapor recovery system are nearly impossible to find visually, we use a smoke machine now to inject a white, non-toxic smoke into the system to see where it is sneaking out. That smoke will show up at the gas cap if that is what is not sealing properly. If you suspect the leak is caused by the gas cap, a new one would be an inexpensive investment to try to solve the problem before spending the money to have your mechanic search further.

Also, look for hose clamps that are rusted off, particularly on the vapor return hose next to the filler tube. Also, take a look at this article related to this issue:

https://www.2carpros.com/articles/evap-system-code-repair