For a couple of months my engine light would come on and I noticed my gas cap no longer clicked when tightening it. At least 10 time, I would tighten the gas cap and the CE would go off in a day or so. I finally took the time to go to the dealer and buy a new cap being 100% sure that was the problem. After 2 weeks and many miles the CE was still on. GRRRR! So I hooked up an OBD reader and it had a code set--don't remember now what it was. I reset the CE and drove for several days and a few hundred miles. Now, it has a P0456 (I do know this was not the code before) (minor leak) and a flashing "F" (for fuel system) indicator on the reader. CE light still not on. Don't know what to think of this. Any thoughts ?

 

If there is no check engine light then the code you read was a "stored" code.
It takes two failed drive cycles for the check engine light to come on
And it takes two drive cycles for it to go off.
The drive cycles are different for different makes and models.

P0456 is a code for a small leak in the Evap emissions recovery system.
A smoke test would help determine where it is and what needs to be done to fix it

I know it is not your car
But it is to let you know what I am talking about ... smoke machine



This one is trying to help you see what other method is possible


Good luck