Honda civic 2003 check engine light on Code p1456

My 2003 civic check engine light is on today. I took it to autozone for a check. It gave me a code of P1456 - it is about the emission system.

I searched on the internet. There are quite a few people who have the same issue. From what people posted, P1456 seems to relate to some kind of gas tank leaking. For someone, it is just loose gas cap, but there could be other reasons.

I made sure my gas cap is tightened. The light is still on. Does anybody here know what could be wrong?

Thanks!