got P0446 engine trouble light

[dna]

My engine light went off earlier this week - read it with my obdii and it gave me P0446 code. I erased the code and drove about 30 minutes without the light coming back. Started the truck later that day and the light was back. Find that some days the light is off and other days it comes back on - sometimes immediately when I start it other days after driving for a while.

Looked it up on google and sounds like a back pressure in gas tank issue. Anyone have experience with this - what should I look at first?

FYI - no issues with the engine when light goes on (still runs perfect).

Thanks