Been chasing this P0451 code in my 2011 F150 for a long time now.... very slowly. Honestly I haven't been terribly motivated since it doesn't affect any functionality on the truck. I believe I've narrowed it down to a fuel tank pressure sensor. Both purge valve and solenoid function but when the tank is at atmospheric pressure its reading a negative value- confirmed its atmospheric pressure by inserting the fuel funnel into the capless fuel filler. Does this sound right? I guess my next step is to hunt down the fuel tank pressure sensor, which I believe it part of a larger tube complex.