Getting code for large evap leak?

There are a lot of places that you can get an EVAP leak. The evap system generally needs to be smoke tested. A shop will fill the system with a low pressure smoke machine designed to find small leaks. Then go through the vehicle to locate anywhere the leak may be coming from.

I have seen technicians use there best educated guess as to what may be causing the leak. Sometimes they get lucky. The Vehicle has to be under certain condtions to run its self test. so if you end up replacing any components yourself, it may take a few test drives for the test to run and confirm the repair. If you do not have an EVAP smoke machine, but are capable of replacing parts. You could pay for a diagnosis and then replace it yourself to save time.

I ran into a similar issue with my Subaru last year. Like you, I tightened the gas cap thinking it was an easy fix, but the code kept popping back up.

The way I found was a smoke test on the EVAP system—EVAP leaks are usually tiny cracks in dry-rotted hoses or O-rings that you simply can't see with the naked eye.

I used a compact smoke tester (a Foxwell ST203 style unit with a built-in pump) so I didn’t need an external compressor and dont need to pay shop labor rates for diagnosis. It pushed smoke through the EVAP lines and the leak showed up right away.

Just a heads-up: once you fix it, you'll need a few drive cycles for the EVAP monitor to clear itself. It's been months now and my CEL is still off.