What does P1456 Evap Emission Control Sys leak mean?

if you try to replace gas cap already, 99% would be your two way valve by the gas tank.

p1456 means you have a leak in your evaporative system which collects the evaporative gases from you fuel and reroutes them back into the intake. you will notice increased fuel usage and possible overheating problems. there is a possibility that it might only be the fuel cap, but it is also possible that it is a much greater problem, as in a hole in the wrong place...

This kind of misinformation is why you can't trust the internet for correct answers.

You would be hard pressed to make a reasonable case for a P1456 causing any kind of overheating. Somewhere your fuel tank is unable to hold the fumes inside without releasing them into the atmosphere, thus causing pollution. That should in no way cause increased fuel use or overheating. A test is available for that concern using nitrogen gas being pressurized in the fuel tank/vapor recovery system to pinpoint the leak so you are not just throwing money at it guessing.