2008 Impala ls P0455

[impala3.5]

I just got the car back in march with 83,000 miles on it. It's an Impala ls 3.5l. At first it was just a tighten gas cap message. Then the cel turned on and gave code p0455, so i swapped out the solonoid thats to the right of the engine under the cover, and cleared the code. Five days later same thing, so i swapped out the gas cap, three trips later it came back. Took it to my buddy and used a gm tech II scanner and this is what it showed.

EVAp vent sol ckt short - not run
EVAP vent sol ckt Open - not run

Evap vent solonoid com not venting.

I'm not too experience with cars, so any suggestions.