Could be a purge solenoid, or a pressure sensor. I would do a visual check on all hoses and connections to start. If you can view live data on a scan tool you can see what the pressure sensor is reading. It should only have vacuum on it when the system is testing for leaks. It vents to the canister which is "purged" by the solenoid into the intake to be burned. It will throw a code if the system won't hold a vaccum, or has a vaccum when its not supposed to.