My 02 infinity qx4 has SES of code p1456 evap small leak detected. could this b a bad ground in the sensor?

I say that because it turns on and off periodically. A couple hundred miles the ses light turns on and for a couple hundred it turns off. My nissan dealer says i need to replace the control valve and entired evap canister. I want to call bullsh*t and say ur robbing me of money. How do i check to see if its a faulty wire? Im sorry im not car literate so please explain carefully. Thank you